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Heavy metal removal in primary sedimentation I. The influence of metal solubility
The Science of the Total Environment, 63 (1987) 231 246 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
231
HEAVY METAL REMOVAL IN PRIMARY S E D I M E N T A T I O N I. THE I N F L U E N C E OF METAL SOLUBILITY
S. KEMPTON, R.M. STERRITT and J.N. LESTER Public Health Engineering Laboratory, Civil Engineering Department, Imperial College, London SW7 2BU (United Kingdom) (Received March 10th, 1986; accepted October 5th, 1986)
ABSTRACT
Heavy metal removal in the primary sedimentation process is primarily influenced by metal solubility and the settleability of insoluble forms. The fraction of the total metal in soluble form in raw sewage was investigated over a range of suspended solids (SS) concentrations and chemical oxygen demand (COD) values, obtained by adding primary sludge and a soluble synthetic sewage, respectively, to raw sewage samples. The soluble fraction in raw sewage was influenced to a greater extent by suspended solids than by COD. Increasing the suspended solids concentration caused marked decreases in the soluble fraction of Cd, Cr, Cu and T1. The addition of soluble heavy metals in order to achieve concentrations typical of mixed industrial]domestic sewage did not in general have a significant effect on solubility. Thus, the removal of heavy metals could be assessed at these elevated concentrations in a pilot scale primary sedimentation plant. The plant was operated at four different influent suspended solids concentrations at each of four different hydraulic loadings covering the range from 0.5 to 5 dry weather flow (DWF) equivalents. Increases in influent suspended solids concentrations caused higher metal removals, whereas increasing the hydraulic loading had the opposite effect. INTRODUCTION
Primary sedimentation of raw sewage is employed to remove most of the settleable solids prior to treatment of the primary effluent by secondary biological processes. During sedimentation, heavy metals associated with the settleable fraction will also be removed. Typically, 40-70% of the Cd, Cr, Cu, Pb and Zn in raw sewage is removed in primary treatment, but removals of Ni and Mn, for example, are significantly lower, being in the region of 20-30% (Oliver and C o s g r o v e , 1974; F e i l e r e t al., 1979; L e s t e r e t al., 1979; S t o v e l a n d e t al., 1979; R o s s i n e t al., 1983). T h e r e m o v a l o f h e a v y m e t a l s i n p r i m a r y s e d i m e n t a t i o n is important for two reasons. Firstly, it reduces the metal loading on the biological stage of treatment, thus reducing the possibility of impairment of treatment e f f i c i e n c y d u e t o m e t a l t o x i c i t y ( B a r t h e t al., 1965). S e c o n d l y , i t c o n t r i b u t e s t o the overall removal efficiency for the treatment works, thus reducing contamination of surface waters to which final effluents are discharged. Metals are only removed if they exist in particulate forms or become adsorbed onto suspended solids and if these solids fractions are settleable (Oliver and C o s g r o v e , 1974). T h u s , i n t e r m s o f p h y s i c o c h e m i c a l s p e c i a t i o n t h e s o l u b l e m e t a l
0048-9697]87]$03.50
© 1987 Elsevier Science Publishers B.V.
232 fraction and solid phase distribution of heavy metals are two important factors which will affect their removal (Lester, 1983). Soluble species of heavy metals are removed with efflciencies consistently less than 1% (Oliver and Cosgrove, 1974). Therefore, any factor which promotes the formation of soluble species, such as the presence of soluble organic metal-binding agents (Rossin et al., 1982) will have a directly adverse effect on removal. Conversely, factors which reduce metal solubility, such as increases in suspended solids concentrations, will tend to increase removal (Rossin et al., 1983). In order to investigate the influence of influent suspended solids, COD, heavy metal concentrations, and surface loading on heavy metal removal in primary sedimentation under controlled conditions it is desirable to amend the raw sewage in order to achieve a range of values for these parameters while at the same time avoiding effects on heavy metal distribution not directly attributable to plant operating parameters. Moreover, in order to determine heavy metal removal at concentrations typical of mixed industrial/domestic sewage, it is necessary to establish that the added metals behave in a similar manner to those already present. The work presented here was undertaken in order to investigate the effects of amending raw sewage with suspended solids, COD and soluble heavy metals on intrinsic metal solubility prior to studying the influence of primary sedimentation operating parameters on metal removal in a pilot scale primary sedimentation pilot plant. A detailed analysis of the effects of individual parameters on metal distributions is presented in a subsequent paper (Kempton et al., 1987). MATERIALSAND METHODS
Primary sedimentation pilot plant The pilot plant used was essentially that described by Rossin et al. (1983) and was located at the Hogsmill Valley Water Pollution Control Works (Thames Water Authority). Sedimentation occurred in two identical humus tanks (KTS Construction Co Ltd., Tamworth, Staffs) which were constructed of glass fibre, thus minimising metal contamination. Each tank had a volume of 0.915 m 3. The upper section had a height of 1.1 m and was circular in cross section with a diameter of 1.0 m. The base of each tank was conical (60°) and 0.8 m in depth. Sewage was introduced to the centre of a tube of height 0.8m and diameter 0.3 m mounted vertically in the centre of each tank. The effluent from each tank passed over twenty-one 45° V-notch weirs to a circular collecting trough from which it was discharged. Prior to entering the sedimentation tanks raw sewage from the works was pumped by a submersible pump fitted with a 20 mm screen to a constant head tank which discharged into a distribution chamber via an adjustable 30° V-notch weir which permitted control of the flow rate. The flow was distributed equally to each tank from the distribution chamber via two adjustable rectan-
233 gular weirs. Prior to entering the tanks, the two flows each passed upwards through a vertically mounted tube fitted with internal baffles which ensured thorough mixing. When amendment of the raw sewage was necessary, the additions were made to this mixing chamber to ensure homogeneity of the settled sewage prior to its entry to the sedimentation tanks. Sample points were located equidistant between each mixing chamber and the inlet to the sedimentation tank (raw sewage), immediately below the effluent outlet (settled sewage) and in separate storage tanks into which the entire contents of the sedimentation tanks were discharged at the end of each experiment (primary sludge). Details regarding the representative sampling of the primary sludge are given elsewhere (Rossin et al., 1983). The sedimentation tanks were operated in parallel, each receiving the same flow but with differing suspended solids concentrations. Thus eight paired experiments were undertaken so that performance at four different influent suspended solids concentrations for each of four different flow rates could be assessed. The flow rates used were 0.055, 0.11, 0.33 and 0.551 s -1, corresponding to 0.5, 1.0, 3.0 and 5.0 DWF. These flow rates were equivalent to surface loadings in the range 6.1-61mday 1 thus, the detention time was 4.6h at 0.5 DWF and 0.5 h at 5 DWF. When suspended solids amendment was required, primary sludge from the sedimentation tanks of the works was collected following a desludging operation and stored in 601 aspirators. The sludge was diluted with primary effluent from the works to form a slurry which was transferred to the sewage flow just after the distribution system and immediately prior to the mixing chamber. In this way, the influent suspended solids concentration could be increased by nominal increments of 150, 300 and 500mg1-1, so that the range of solids concentrations studied was approximately 400-1000 mg l-1. Amendment of the raw sewage with heavy metals was achieved by pumping concentrated standard solutions (unacidified) into the system at the point where the sewage passed over the V-notch weir. These solutions were prepared in such a way that the nominal final concentrations of heavy metals in the raw sewage were 0.5mgl 1 for Zn, 0.1mgl -~ for Co, Cr, Cu, Mn, Ni and Pb, 0.05mg1-1 for Mo and 0.01 mgl -~ for Ag and Tl. These concentrations are typical of those found in mixed industrial/domestic wastewaters. The duration of each experiment was dependent on the flow rate. The sedimentation tanks were filled with raw sewage, the flow was started and solids amendment, where appropriate, was initiated. One retention time was allowed to elapse prior to the addition of heavy metals. After one further retention time, sampling was started and the pilot plant continued in operation for a period equivalent to 10 retention times. Thus, at one dry weather flow (DWF), for example, sampling was continued for 24 h. The sampling frequency was also dependent on the flow rate, and was designed so that flow-proportional composites could be obtained. Each composite was made up from six individual samples. The number of composites corresponded to the number of retention times elapsed during each experiment (i.e. 10 composites).
234 TABLE 1 Analytical data for heavy metal determinations (including data from Sterritt and Lester (1980) and Kempton et al. (1982)) Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Typical conc. in sewage (]~gl 2)
Typical analytical working range (pgl 2)
Relative standard deviation (%)a
Reported detection limit (ttgl-1) b
0.5 4 3 100 500 15 5 100 50 <1 100
1-2O 1 20 1 10 10-100 5--50 1 ~ 100 2 20 2-20 20-200 5-50 1 10 20-200
5 7 15 5 2 8 9 6 7 5 11 3
0.005 0.2 0.003 0.02 0.01 0.02 0.01 0.02 0.1 0.05 0.1 0.001
a Defined as standard deviation divided by mean concentration, calculated from five replicate samples. bInstrument manufacturers' data.
Analytical techniques Suspended solids concentrations (SS) and chemical oxygen demand (COD) were determined by U.K. standard methods (Government of Great Britain, 1977, 1984). For all experiments involving heavy metals, borosilicate ('Pyrex') glass, polytetrafluoroethylene (PTFE) or polypropylene apparatus was used. This was cleaned with a proprietary detergent, rinsed thoroughly and soaked for 24 h in 10% v/v Analar grade nitric acid prior to use. Samples for heavy metal analysis were acidified to 1% (v/v) ~Aristar' grade nitric acid immediately upon collection, except where they were filtered. Filtration was performed on-site and as soon as possible after sample collection, prior to acidification of the filtrates to 1% (v/v) nitric acid. Heavy metals were determined by atomic absorption spectrophotometry. Flame and flameless atomic absorption determinations were made using a Perkin -Elmer model 5000 double beam instrument fitted with standard burner heads and an HGA-500 graphite atomiser respectively. Details of the analytical procedures and sample pretreatments have been reported previously for Cd, Cr, Cu, Ni, Pb, Zn, Ag, Co, Mn and Mo (Sterritt and Lester, 1980) and Bi and T1 (Kempton et al., 1982). Precision data, normal analytical working ranges and reported detection limits for these analytical procedures are shown in Table 1.
235 TABLE 2 The regression of soluble metal on total metal concentrations in raw sewage Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Regression coefficient (b) 0.11 0.09 0.43 0.84 0.91 0.32 0.72 0.68 0.69 0.02 0.50 0.56
Standard deviation (/~gl 1) 1.8 4.0 5.2 4.9 6.6 48 38 150 15 2.1 2.2 90
Range of soluble metal concentration (pgl 1) 0.5-9.2 1.2-4.5 0.5~43 14~8 1.2-400 30-340 20-750 150~830 7.5~400 5-17 6.2 27 200-2000
t-test level of significance b>0
b
0.05 0.01 0.01 0.01 0.001 0.01 0.001 0.10 0.001 0.02 0.01 0.001
0.001 0.001 0.002 NS 0.01 0.001 0.01 NS 0.01 0.001 0.01 0.001
Batch experiments S c r e e n e d and c o m m i n u t e d r a w sewage to be used in b a t c h tests was o b t a i n e d from the Hogsmill V a l l e y W o r k s at a p o i n t just prior to its e n t r y to the p r i m a r y s e d i m e n t a t i o n tanks. It was t r a n s p o r t e d to the l a b o r a t o r y and used for experim e n t a t i o n w i t h i n 12h to p r e v e n t significant d e t e r i o r a t i o n due to microbial activity. Since the r a w sewage e n t e r i n g the Hogsmill w o r k s is almost entirely of domestic origin, it was n e c e s s a r y to add h e a v y metals to it, in o r d e r to i n v e s t i g a t e the effects of metal c o n c e n t r a t i o n and to o b t a i n c o n c e n t r a t i o n s in the influent to the pilot p l a n t typical of mixed i n d u s t r i a l / d o m e s t i c sewage. E l e v a t e d c o n c e n t r a t i o n s of h e a v y metals in the r a w sewage were o b t a i n e d by a d d i n g small aliquots (20-200 pl) of unacidified m e t a l stock solutions to 200 ml samples w h i c h were s u b s e q u e n t l y allowed to equilibrate with mixing for 3 h p r i o r to c o n t i n u i n g with f r a c t i o n a t i o n procedures. The same sewage samples were also amended, w h e r e appropriate, by the s i m u l t a n e o u s addition of prim a r y sludge (2-4 ml) in order to i n c r e a s e the suspended solids c o n c e n t r a t i o n by i n c r e m e n t s of 1 5 0 m g l -~, or by the addition of 1.5-3ml of a c o n c e n t r a t e d s y n t h e t i c sewage ( S t o v e l a n d and Lester, 1980) to i n c r e a s e the soluble COD by i n c r e m e n t s of a p p r o x i m a t e l y 150 mg l- 1.
Metal fractionation H e a v y m e t a l c o n c e n t r a t i o n s were d e t e r m i n e d in the filtrate o b t a i n e d after filtration of a 10ml subsample t h r o u g h a 0.2#m pore size cellulose a c e t a t e m e m b r a n e filter m o u n t e d in a v a c u u m filtration manifold (Amicon Ltd., Stone-
236 house, Gloucs.). Cleaning and pretreatment of the filters was done in a manner similar to that described by Laxen and Chandler (1982). Metals passing the 0.2 pm filters were taken to be soluble (Laxen and Chandler, 1982), although the use of the term 'solubility' in this paper does not imply any thermodynamic significance. RESULTS
Effects of total metal concentration on the proportion of filterable metal A batch test experiment was designed in order to assess the variation in heavy metal solubilities over a range of total concentrations, and to determine whether total metal concentration and filterable metal concentration in raw sewage were related. Six samples of raw sewage (200 ml) were used: the first remained unamended, containing indigenous heavy metal concentrations as a control. To five other samples, specific volumes of stock solutions of heavy metals were added so as to produce five amended sewages with metal concentrations spread over an approximately 10-fold range. Small volumes of stock solutions were added to avoid dilution of the sewage sample and also to maintain a constant pH. The results are presented in Table 2 for the regression of filterable metal concentration on total metal concentration. A t-test was used to evaluate the significance of the data. A regression coefficient not significantly different from zero would be indicative of a constancy of the concentration of soluble metal with increasing added total metal concentration. A regression coefficient not significantly different from unity would indicate that all of the added metal remained in the soluble form. The data in Table 2 shows that, with the exception of Co and Mo, 1 > b > 0 for all metals with a generally high level of significance. The standard deviation quoted refers to the variation in soluble metal concentrations. Thus, a relatively constant fraction of each metal was in the soluble form irrespective of the total concentration. It was concluded, therefore, that in order to obtain the requisite concentration of each heavy metal in raw sewage for experimental purposes, amendment with stock solutions could be used without having a major effect on the fraction of the total metal existing in a soluble form.
The influence of suspended solids and soluble COD concentrations on soluble metal concentrations Batch experiments were performed in order to assess the interaction of suspended solids and soluble COD and their individual and combined effects on the concentrations of metals in the soluble form. The experiments were designed in such a way that the influence of three different suspended concentrations could be examined at each of three different COD concentrations, giving a total number of samples of nine. This approach permitted the determination of any
237 TABLE 3 P e r c e n t a g e solubilities of toxic e l e m e n t s at v a r y i n g solids a n d COD c o n c e n t r a t i o n s for i n d i g e n o u s a n d i n c r e a s e d toxic e l e m e n t c o n c e n t r a t i o n s Metal
COD ( m g l ~)
S u s p e n d e d solids c o n c e n t r a t i o n (rag 1-~) M e t a l s added 475
590
Metals not added 760
475
590
760
Ag
170 330 475
2.5 2.4 2.9
0.8 1.6 2.0
0.9 1.1 0.8
5.8 4.5 4.1
5.5 2.6 4.1
2.8 2.6 2.2
Cd
170 330 475
62.7 68.7 51.9
34.5 37.6 49.0
38.0 32.3 24.6
44.2 76.7 62.9
65.8 54.5 55.6
52.8 45.2 48.6
Co
170 330 475
68.9 71.1 70.5
66.3 58.5 66.0
60.9 64.0 54.9
ND ND ND
ND ND ND
ND ND ND
Cr
170 330 475
59.8 60.8 57.6
32.3 31.6 34.5
21.3 25.0 26.0
4.6 6.3 5.1
6.4 5.7 11.9
9.5 ].5 6.8
Cu
170 330 475
22.5 22.3 21.0
6.4 6.7 13.5
5.2 8.2 6.3
11.2 14.0 16.0
5.8 6.3 7.2
3.4 4.7 5.1
Mn
170 330 475
66.5 71.2 60.7
64.1 64.6 72.2
67.4 70.6 64.1
53.2 57.3 47.4
53.5 52.0 53.6
43.5 49.4 50.0
Mo
170 330 475
66.7 68.0 52.7
62.0 72.4 56.1
58.6 51.7 47.2
ND ND ND
ND ND ND
ND ND ND
Ni
170 330 475
79.2 74.4 67.4
65.2 64.6 75.9
67.5 64.5 62.5
ND ND ND
ND ND ND
ND ND ND
Pb
170 330 475
16.4 11.4 10.4
6.9 8.0 11.0
12.3 7.7 8.0
7.4 18.6 8.3
12.7 6.5 9.6
T1
170 330 475
59.3 52.9 41.9
18.0 15.2 18.0
14.0 15.4 13.3
ND ND ND
ND ND ND
ND ND ND
Zn
170 330 475
21.2 23.6 30.4
22.1 16.2 27.3
26.6 27.2 22.2
50.0 35.3 46.7
83.3 85.0 54.2
29.2 20.0 58.3
6.5 3.4 3.3
238 RAW SEWAGE (sl)
Filtration
FILTERED RAW SEWAGE (S2)
Iteavv metal
add i t i/in
METAL-DOSED RAW SEWAGE (ss)
[ F [I [ rat inn
RAW ~EWAGE FILTRATE ($6) Fig. I. Experimental m e t h o d h e a v y metal removal.
Heavy' metal addition
METAL-DOSED RAW SEWAGE (s3)
I F] I trat ion
REFILTER~:D METAL-DOSED FILTRATE ($4) used to assess the contribution of adsorption a n d precipitation to
possible interactive effects between the two parameters. Equal quantities of metal stock solutions were added to each sewage sample prior to equilibration for 3 h followed by filtration and determination of the soluble metal. Similar experiments were performed using the indigenous metal concentrations (i.e. those in Table 2) where possible. Table 3 shows the percentage solubilities of metals in each sample at varying solids and COD concentrations for added and indigenous metal concentrations. No results are presented for Bi, as every filtrate contained less than 0.0005 mg l- 1. From the data presented in Table 3 several metals appeared to be influenced by solids concentrations, a decrease in filterable metal corresponding to an increase in solids concentration. These metals were Ag, Cd, Cr, Cu and T1. The soluble metal fraction did not, however, appear to be markedly affected by COD, although in the case of copper an increase in COD was accompanied by increasing filterable metal concentration. These data suggested that soluble organic ligands may play a minor role in determining the proportion of metal in the soluble form in raw sewage.
239 TABLE 4 Potential removals of heavy metals as influenced by adsorption and precipitation in raw sewage Metal
Fraction of total metal (%) Precipitation calculated from $3 - $4 x I00 $3
Total Adsorption calculated column 3 from minus column 2 $5 - S6 x 100 $5
Total removal from Eqn (1)
Adsorption column 5 minus column 2
89 89 69 41 23 81 49 4 22 94 43 73
76 89 88 41 12 100 0 4 22 93 43 100
65 37 88 41 10 89 0 2 22 22 43 58
-
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
11 52 0 0 2 11 4 2 0 71 0 43
-
78 37 69 41 21 70 45 2 22 24 43 31
Adsorption and Precipitation Samples of r a w sewage were used, either filtered or unfiltered, to assess the e x t e n t of a d s o r p t i o n of metals by the solids and p r e c i p i t a t i o n in a solids-free sample, in o r d e r to d i s t i n g u i s h b e t w e e n two m a j o r r e m o v a l m e c h a n i s m s for e a c h metal. By e m p l o y i n g sewage samples with a n d w i t h o u t h e a v y m e t a l additions and filtering b o t h before and after s u c h additions, the p o t e n t i a l r e m o v a l s by p r e c i p i t a t i o n a n d by a s s o c i a t i o n with solids were investigated. The e x p e r i m e n t a l scheme used is r e p r e s e n t e d in Fig. 1, and the results are s h o w n in Table 4. F r o m the samples d e n o t e d S1 and $2 in Fig. 1, the soluble metal c o n c e n t r a t i o n and h e n c e m a x i m u m p o t e n t i a l r e m o v a l of metals was calculated. H o w e v e r , to enable a more direct c o m p a r i s o n b e t w e e n different metals t h e i r c o n c e n t r a t i o n s were increased to similar levels by the addition of s t o c k solutions and h e n c e samples $5 and $4 were used to c a l c u l a t e the p o t e n t i a l r e m o v a l s at these elevated c o n c e n t r a t i o n s . Samples $3 and $4 were used to estimate p o t e n t i a l r e m o v a l due to p r e c i p i t a t i o n m e c h a n i s m s alone. Thus, by s u b t r a c t i n g r e m o v a l due to p r e c i p i t a t i o n from t o t a l removal, a figure was o b t a i n e d w h i c h could be e q u a t e d to r e m o v a l by a d s o r p t i o n o n t o the solid phase. F u r t h e r c a l c u l a t i o n s were performed to d e t e r m i n e the r e m o v a l percent a g e of added metals from the following equation: R e m o v a l (%)
=
($5 - S1) - ($6 - $2) x 100 ($5 - S1)
(1)
240 TABLE 5 Influence of suspended solids c o n c e n t r a t i o n s and hydraulic loading on solids removal (%) Hydraulic loading (DWF)
0.5 1.0 3.0 5.0 Correlation coefficient a Level of significance
Influent suspended solids c o n c e n t r a t i o n (mgl 1) 405
553
733
888
70 62 47 56 - 0.67 NS
74 63 69 55 0.05 NS
79 64 68 60 - 0.71 NS
77 65 65 66 - 0.55 NS
Mean
Correlation coefficient a
Level of significance
75 64 62 59
0.86 1.00 0.64 0.91
0.10 0.001 NS 0.05
a Based on arcsin t r a n s f o r m a t i o n of removal data.
This gave the total removal of added metals and hence removal due to adsorption could be calculated as above. Sample $2 gave inconsistent results for Cd, Cu and Zn; these were probably due to contamination as a result of their initially very low concentrations and values from sample $6 were substituted. Although the estimated potential removal by adsorption varied according to the method of calculation, the relative proportions of removal by precipitation and adsorption were consistent, except in the case of Mn and Zn. Therefore, it was possible to predict which metals would be influenced to the greatest extent by adsorption or precipitation and whether the solid or soluble phase would be the dominant factor in effecting removal. The extent to which this potential removal actually occurs will depend, however, on the settleability of the particulates formed. The behaviour of Bi and Pb appeared to be primarily dominated by precipitation; Ag, Cd, Co, Cu, Cr, Mn, Ni and T1 removals were controlled by adsorption and Zn and Mo removals were influenced by both factors.
Influence of solids and surface loading on solids removal efficiency At each of four hydraulic loadings and four influent suspended solids concentrations, the suspended solids removal efficiencies were calculated. These are shown in Table 5. The arcsin transformation was used to normalise the percentage solids removal data, prior to calculation of the correlations betJ ween solids removal and influent suspended solids concentration and hydraulic loading. It is apparent that the solids removal efficiencies increased with an increasing influent solids concentration and tended to decrease with an increasing hydraulic loading. However, none of the correlation coefficients for the latter effect were significant, indicating that the major influence is influent suspended solids concentration.
241 TABLE 6 M e t a l solids a n d copper c o n c e n t r a t i o n s in i n f l u e n t d i s c r e t e a n d c o m p o s i t e s a m p l e s C o n c e n t r a t i o n (mg1-1) Discrete sample values Suspended solids
Mean
Composite
534
524
226
498
352
320
416
392
198 318 388 268 144 510 286 302
338 348
350 334 144 88 398 664 232 1174
326 220 486 194 352 642 316 274
292 346 208 552 606 526 462 464
162 412 912 98 790 426 540 394
278 330 356 258 415 564 365 509
276 344 428 302 436 578 370 556
388
409
0.156 0.112 0.232 0.084 0.128 0.455 0.255 0.310 0.220
0.255 0.157 0.195 0.167 0.193 0.250 0.300 0.243 0.292
0.256 0.156 0.206 0.176 0.210 0.252 0.270 0.252 0.280
Overall m e a n
0.228
0.229
346 344 618 352 444
Overall mean Copper 0.118 0.198 0.228 0.172 0.168 0.290 0.220 0.260
0.306 0.162 0.210 0.138 0.244 0.172 0.320 0.225 0.255
0.306 0.180 0.198 0.120 0.116 0.186 0.320 0.145 0.355
0.276 0.192 0.146 0.280 0.180 0.202 0.345 0.220 0.400
0.230 0.176 0.188 0.154 0.320 0.316 0.300 0.340 0.260
Influence of solids concentrations and hydraulic loading on heavy metal removal Factors which may influence the removal efficiencies of heavy metals during primary sedimentation were also investigated during operation of the pilot plant at four different influent solids concentrations at each of four different hydraulic loadings. During these experiments it was found necessary to prepare composite samples for analysis, and so the validity of doing this was assessed by comparing results from both individual discrete samples and composite samples. The outcome of these experiments for suspended solids and for copper determinations is shown in Table 6. The final two columns in Table 6 were compared using a t-test, and were not found to be significantly different at the 5% level of significance. The practise of compositing samples was therefore considered acceptable. Removal efficiencies as percentages have been summarised in Table 7 for the 12 metals studied. Throughout these experiments mean removal efficiencies varied from 4 to 54%. It is apparent from Table 7 that both solids concentration and flow rate exerted an influence on the removals of the heavy metals. Generally, increased
242 TABLE 7
The influence of influent suspended solids concentration and hydraulic loading on heavy metal removal (mean removal efficiency (%)) Metal
Suspended solids concentrations
Hydraulic loading (DWF)
(mgl-')
Ag Bi Cd Co Cr Cu
Mn Mo Ni Pb
T1 Zn
405
553
733
888
32 10 17 15 16 31 22 4 23 36 23 32
41 19 25 24 24 40 27 11 23 42 27 37
37 8 25 20 14 38 25 12 19 38 35 33
52 19 24 34 28 49 30 13 36 52 54 52
0.5
1.0
3.0
5.0
54 16 24 30 29 54 36 13 30 52 42 46
44 17 26 28 23 23 21 14 22 40 38 36
35 7 17 20 28 43 23 6 15 42 31 39
29 16 24 14 11 38 23 7 34 34 29 32
removals were observed with an increasing solids loading and decreased removals were observed for increasing flow rates. Correlation coefficients were calculated for heavy metal removal efficiencies against influent solids concentrations, for each flowrate, again using the arcsin transformation of the heavy metal removal efficiency data. These results are summarised in Table 8. With the exception of Bi, Cr and Cu the correlation between metal removal and influent suspended solids concentration at 1 DWF was significant with P < 0.05. Mass balances Mass balances were calculated for each metal during each experimental run of the pilot plant in order to indicate the reliability of the nature of sampling, particularly in the case of the sludge suspension, which was sampled only once at the end of each experiment. Mass balances were calculated from the following equation: Mass balance
=
[ VI " M I ]
WE'ME] + [½"Ms]
×
100(%)
(2)
where V1 = total influent volume (1), VE = total effluent volume (1), Vs = sludge suspension volume (1), M[ = heavy metal concentration in influent (rag l- ~), M E = heavy metal concentration in effluent (mg l-~), Ms = heavy metal concentration in sludge suspension (rag 1 1). The results of the mass balance calculations are presented in Table 9. Generally the mass balances for each metal at each experiment were accept-
243 TABLE 8 Correlations a between heavy metal removal and influent suspended solids concentrations at different hydraulic loadings Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Hydraulic loading (DWF) 0.5
1.0
3.0
0.82 0.23 - 0.14 0.07 0.18 0.81 0.14 0.96* - 0.01 0.26 0.71 0.22
0.88* - 0.29 0.97* 0.97* 0.65 0.65 0.92* 0.56 0.99* 0.90* 0.90* 0.98*
0.75 0.25 0.84 0.80 0.75 0.79 0.81 0.26 0.80 0.59 0.84 0.82
5.0 0.45 0.93* 0.05 0.95* 0.74 0.98* 0.79 0.59 0.18 0.74 0.69 0.85
Based on arcsin transformation. * Statistically significant at p = 0.05. able. By c a l c u l a t i n g m e a n v a l u e s from the mass b a l a n c e s d a t a for each metal and for each e x p e r i m e n t a l r u n an overall i n t e r p r e t a t i o n of individual mass b a l a n c e s was obtained. Thus, m e a n mass b a l a n c e s for each metal d u r i n g all e x p e r i m e n t a l r u n s gave v a l u e s from 93 to 106% and for all metals e a c h experim e n t a l r u n gave v a l u e s from 84 to 148% with a c o m m o n m e a n for all the d a t a of 100%. These results i n d i c a t e t h a t the a n a l y t i c a l m e t h o d o l o g y for every metal studied was reliable, and t h a t d e v i a t i o n s from the 100% expected v a l u e were l a r g e l y due to the effects of process o p e r a t i o n . In p a r t i c u l a r , the h o m o g e n e i t y of the sludge s u s p e n s i o n was critical in p r o d u c i n g good mass balances. No distinct t r e n d s were a p p a r e n t for m e a n mass b a l a n c e s in terms of solids loadings at each flow rate. DISCUSSION
The r e m o v a l of h e a v y metals d u r i n g w a s t e w a t e r t r e a t m e n t is an advant a g e o u s c o n s e q u e n c e of the existence of settleable particles in the influent and c o n v e r s i o n of soluble and non-settleable to settleable forms d u r i n g t r e a t m e n t . Thus, the p h y s i c a l r e m o v a l of settleable forms of h e a v y metals is largely influenced by the solids removal. D u r i n g the c o u r s e of the p r i m a r y sedimentation pilot p l a n t experiments, a c c e p t a b l e r e m o v a l s of suspended solids from the raw sewage influent were a c h i e v e d a n d it was a p p a r e n t t h a t the s e d i m e n t a t i o n t a n k s were p e r f o r m i n g s a t i s f a c t o r i l y in c o m p a r i s o n with the predictions of t h r e e t h e o r e t i c a l models. It is evident t h a t the a v e r a g e r e m o v a l efficiencies r e c o r d e d in these e x p e r i m e n t s c o m p a r e well with the v a l u e s predicted for
244 TABLE 9 M a s s b a l a n c e s for h e a v y m e t a l s d u r i n g t h e p r i m a r y s e d i m e n t a t i o n s t u d y Hydraulic loading (DWF)
Suspended solids ( m g l 1)
Ag
Bi
Cd
Co
Cr
Cu
Mn
Mo
Ni
Pb
T1
Zn
Mean
0.5
405 553 733 888
114 174 135 110
81 160 85 98
97 165 91 89
116 161 98 111
122 164 82 114
108 144 115 116
114 130 99 93
103 104 111 121
151 116 95 150
106 174 82 116
97 127 86 95
104 159 88 129
109 148 97 112
1.0
405 553 733 888
95 85 83 82
111 58 90 81
99 98 105 100
97 97 101 98
106 98 92 80
115 91 94 83
107 99 85 74
99 104 111 103
100 91 99 98
101 73 74 61
115 102 103 86
100 67 95 83
104 89 94 86
3.0
405 553 733 888
83 87 89 130
96 86 86 109
96 91 90 109
93 90 93 128
86 102 93 103
81 83 75 148
76 80 72 111
87 102 105 89
85 82 93 95
72 77 73 142
89 73 71 130
103 95 85 147
87 87 85 120
5.0
405 553 733 888
76 71 75 119
99 97 102 105
106 98 99 95
77 89 100 101
99 80 99 100
94 75 89 114
86 79 78 98
94 102 95 101
125
85 69 66 112
105 77 97 113
84 88 101 122
94 84 92 107
100
96
102
103
101
102
93
102
93
98
103
100
Mean
M a s s b a l a n c e s (%)
109
106
full-scale tanks by the three theoretical models, the results lying within the range of the values at lower hydraulic loadings derived by Smith (1969) and Tebbutt and Christoulas (1975) and the perhaps somewhat optimistic values predicted for higher hydraulic loadings which have been discussed previously (Anderson, 1981). Similarly, the suspended solids removal efficiencies obtained at varying influent suspended solids loading, which was a major variable studied during these experiments, compare favourably with those derived previously (Tebbutt and Christoulas, 1975; Anderson, 1981) as do the values obtained for non-settleable solids. Both solids loading and hydraulic loading appear to influence the removal efficiencies of solids. Optimum removals were generally obtained for higher solids loadings and for the lowest flowrate. It would appear, however, that a decline in solids removal occurred at the upper extreme of solids loading in some cases. At flows equivalent to 0.5 and 3 DWF, the maximum solids removal occurred at an intermediate solids loading, indicating that the plant may have been overloaded. Therefore, in practice, the operation of primary sedimentation in terms of solids and hydraulic loading was critical, and clearly the greater precision in controlling these parameters in keeping with the design limits would give optimum performance in terms of solids removal. The heavy metal mass balance calculations for the primary sedimentation studies indicated the greatest variation in observed removal efficiencies to be
245 due to the effects of operational parameters rather than analytical or sampling error. Average removals were 41% Ag, 14% Bi, 23% Cd, 23% Co, 20% Cr, 39% Cu, 26% Mn, 10% Mo, 25% Ni, 42% Pb, 35% T1, and 38% Zn. These results generally agree with reported values for removal efficiencies during primary sedimentation (Oliver and Cosgrove, 1974; Rossin et al., 1983), although the variation in removal efficiencies has resulted in relatively poor mean values in some cases. The variation in influent toxic element concentrations was minimised by continuous addition of stock solutions. However, some variation was unavoidable since raw sewage and primary sludge quality was dependent on external factors. The sampling system adopted after comparing discrete with composite sampling was intended to minimise any external factors so enabling experiments to be performed on a comparative basis. The fairly wide variations in heavy metal removal efficiencies suggested that the operating parameters for primary sedimentation can significantly influence heavy metal removal. These experimental results have demonstrated the influence of both solids loading and flowrate on heavy metal removal efficiencies. The purely physical processes of settling particulate forms of heavy metals would be expected to remove very little of the soluble forms of heavy metals (Oliver and Cosgrove, 1974). Therefore, as expected, the removals of heavy metals were related to solids removal. There were some significant correlations between heavy metals removals and suspended solids concentrations. Since solids removals were found to be influenced to a greater extent by influent solids concentraion than by overflow rate, it is not suprising that heavy metal removal efficiencies were influenced in this way. This suggests t h a t the distributions of toxic elements and particulate matter were similar if not coincident. This possibility is investigated in a subsequent paper (Kempton et al., 1987). CONCLUSIONS Among the factors which would be expected to influence heavy metal solubility in raw sewage, the suspended solids concentration had the greatest effect on Ag, Cd, Cr, Cu and T1, with reductions in metal solubility of 50% or more accompanying increases in suspended solids of up to approximately 50%. The percentage solubilities of these metals were also largely unaffected by increases in added metal concentrations. Metal solubility was not significantly influenced by soluble COD, indicating that the solid phase was the dominant controlling factor. Much of the metal in the solid phase was adsorbed onto the suspended solids. The formation of precipitates was significant only in the case of Bi, Pb and Zn. The pilot scale primary sedimentation plant performed according to predictions of full scale plants. Solids removal efficiency was in direct proportion to solids loading, whereas increasing the hydraulic loading caused marked reductions in solids removal. These factors were reflected in the heavy metal removal efficiencies obtained, the closest correlations between metal removal and solids loading were obtained for several elements, particularly those of low solubility.
246 ACKNOWLEDGEMENTS The authors acknowledge the generous cooperation of Mr E Eves, Manager o f t h e H o g s m i l l V a l l e y W a t e r P o l l u t i o n C o n t r o l W o r k s . O n e o f u s ( S K ) is grateful to the Science and Engineering Research Council for the award of a postgraduate studentship. REFERENCES Anderson, J.A., 1981. Primary sedimentation of sewage. Water Pollut. Control, 3: 413-420. Barth, E.F., M.B. Ettinger, B.V. Salotto and G.N. McDermott, 1965. Summary report on the effects of heavy metals on the biological treatment processes. J. Water Pollut. Control Fed., 37: 86-96. Feiler, H.D., A.S. Vernick and P.J. Stotch, 1979. Fate of priority pollutants in POTW's. Municipal Sludge Management, Proc. 8th National Conf., Miami, Information Transfer Inc., Silver Springs, MD, pp. 72-81. Government of Great Britain, 1978. Chemical Oxygen Demand (Dichromate Value) of Polluted and Waste Waters 1977. Methods for the Examination of Waters and Associated Materials. HMSO, London, 14 pp. Government of Great Britain 1984. Suspended Settleable and Total Dissolved Solids in Waters and Effiuents 1980. Methods for the Examination of Waters and Associated Materials. HMSO, London, 27 pp. Kempton, S., R.M. Sterritt and J.N. Lester, 1982. Atomic absorption spectrophotometric determination of antimony arsenic, bismuth, tellurium, thallium and vanadium in sewage sludge. Talanta, 29: 675~78. Kempton, S., R.M. Sterritt and J.N. Lester, 1987. Heavy metal removal in primary sedimentation II. The influence of metal speciation and particle size distribution. Sci. Total Environ., 63: 247 258. Laxen, D.P.H. and I.M. Chandler, 1982. Comparison of filtration techniques for size distribution in fresh waters. Anal. Chem., 54: 1350-1355. Lester, J.N., 1983. Significance and behaviour of heavy metals in waste water treatment processes. I. Sewage treatment and effiuent discharge. Sci. Total Environ., 30: 1-44. Lester, J.N., R.M. Harrison and R. Perry, 1979. The balance of heavy metals through a sewage treatment works. I. Lead, cadmium and copper. Sci. Total Environ., 12: 13-23. Oliver, B.G. and E.G. Cosgrove, 1974. The efficiency of heavy metal removal by a conventional activated sludge treatment plant. Water Res., 8: 86~874. Rossin, A.C., R.M. Sterritt and J.N. Lester, 1982. The influence of process parameters on the removal of heavy metals in activated sludge. Water Air, Soil Pollut. Rossin, A.C., R.M. Sterritt and J.N. Lester, 1983. The influence of flow conditions on the removal of heavy metals in the primary sedimentation process. Water, Air, Soil Pollut., 19: 105-121. Smith, R., 1969. Preliminary design of wastewater treatment system. J. Sanit. Eng. Div., Am. Soc. Civ. Eng., 95: 117. Sterritt, R.M. and J.N. Lester, 1980. Atomic absorption spectrophotometric analysis of the metal content of waste water samples. Environ. Technol. Lett., 1: 402-417. Stoveland, S. and J.N. Lester, 1980. A study of the factors which influence metal removal in the activated sludge process. Sci. Total Environ., 16:37 54. Stoveland, S., M. Astruc, J.N. Lester and R. Perry, 1979. The balance of heavy metals through a sewage treatment works. II. Chromium, nickel and zinc. Sci. Total Environ., 12: 25-34. Tebbutt, T.H.Y. and D.G. Christoulas, 1975. Performance relationships for primary sedimentation. Water Res. 9: 347-356.
231
HEAVY METAL REMOVAL IN PRIMARY S E D I M E N T A T I O N I. THE I N F L U E N C E OF METAL SOLUBILITY
S. KEMPTON, R.M. STERRITT and J.N. LESTER Public Health Engineering Laboratory, Civil Engineering Department, Imperial College, London SW7 2BU (United Kingdom) (Received March 10th, 1986; accepted October 5th, 1986)
ABSTRACT
Heavy metal removal in the primary sedimentation process is primarily influenced by metal solubility and the settleability of insoluble forms. The fraction of the total metal in soluble form in raw sewage was investigated over a range of suspended solids (SS) concentrations and chemical oxygen demand (COD) values, obtained by adding primary sludge and a soluble synthetic sewage, respectively, to raw sewage samples. The soluble fraction in raw sewage was influenced to a greater extent by suspended solids than by COD. Increasing the suspended solids concentration caused marked decreases in the soluble fraction of Cd, Cr, Cu and T1. The addition of soluble heavy metals in order to achieve concentrations typical of mixed industrial]domestic sewage did not in general have a significant effect on solubility. Thus, the removal of heavy metals could be assessed at these elevated concentrations in a pilot scale primary sedimentation plant. The plant was operated at four different influent suspended solids concentrations at each of four different hydraulic loadings covering the range from 0.5 to 5 dry weather flow (DWF) equivalents. Increases in influent suspended solids concentrations caused higher metal removals, whereas increasing the hydraulic loading had the opposite effect. INTRODUCTION
Primary sedimentation of raw sewage is employed to remove most of the settleable solids prior to treatment of the primary effluent by secondary biological processes. During sedimentation, heavy metals associated with the settleable fraction will also be removed. Typically, 40-70% of the Cd, Cr, Cu, Pb and Zn in raw sewage is removed in primary treatment, but removals of Ni and Mn, for example, are significantly lower, being in the region of 20-30% (Oliver and C o s g r o v e , 1974; F e i l e r e t al., 1979; L e s t e r e t al., 1979; S t o v e l a n d e t al., 1979; R o s s i n e t al., 1983). T h e r e m o v a l o f h e a v y m e t a l s i n p r i m a r y s e d i m e n t a t i o n is important for two reasons. Firstly, it reduces the metal loading on the biological stage of treatment, thus reducing the possibility of impairment of treatment e f f i c i e n c y d u e t o m e t a l t o x i c i t y ( B a r t h e t al., 1965). S e c o n d l y , i t c o n t r i b u t e s t o the overall removal efficiency for the treatment works, thus reducing contamination of surface waters to which final effluents are discharged. Metals are only removed if they exist in particulate forms or become adsorbed onto suspended solids and if these solids fractions are settleable (Oliver and C o s g r o v e , 1974). T h u s , i n t e r m s o f p h y s i c o c h e m i c a l s p e c i a t i o n t h e s o l u b l e m e t a l
0048-9697]87]$03.50
© 1987 Elsevier Science Publishers B.V.
232 fraction and solid phase distribution of heavy metals are two important factors which will affect their removal (Lester, 1983). Soluble species of heavy metals are removed with efflciencies consistently less than 1% (Oliver and Cosgrove, 1974). Therefore, any factor which promotes the formation of soluble species, such as the presence of soluble organic metal-binding agents (Rossin et al., 1982) will have a directly adverse effect on removal. Conversely, factors which reduce metal solubility, such as increases in suspended solids concentrations, will tend to increase removal (Rossin et al., 1983). In order to investigate the influence of influent suspended solids, COD, heavy metal concentrations, and surface loading on heavy metal removal in primary sedimentation under controlled conditions it is desirable to amend the raw sewage in order to achieve a range of values for these parameters while at the same time avoiding effects on heavy metal distribution not directly attributable to plant operating parameters. Moreover, in order to determine heavy metal removal at concentrations typical of mixed industrial/domestic sewage, it is necessary to establish that the added metals behave in a similar manner to those already present. The work presented here was undertaken in order to investigate the effects of amending raw sewage with suspended solids, COD and soluble heavy metals on intrinsic metal solubility prior to studying the influence of primary sedimentation operating parameters on metal removal in a pilot scale primary sedimentation pilot plant. A detailed analysis of the effects of individual parameters on metal distributions is presented in a subsequent paper (Kempton et al., 1987). MATERIALSAND METHODS
Primary sedimentation pilot plant The pilot plant used was essentially that described by Rossin et al. (1983) and was located at the Hogsmill Valley Water Pollution Control Works (Thames Water Authority). Sedimentation occurred in two identical humus tanks (KTS Construction Co Ltd., Tamworth, Staffs) which were constructed of glass fibre, thus minimising metal contamination. Each tank had a volume of 0.915 m 3. The upper section had a height of 1.1 m and was circular in cross section with a diameter of 1.0 m. The base of each tank was conical (60°) and 0.8 m in depth. Sewage was introduced to the centre of a tube of height 0.8m and diameter 0.3 m mounted vertically in the centre of each tank. The effluent from each tank passed over twenty-one 45° V-notch weirs to a circular collecting trough from which it was discharged. Prior to entering the sedimentation tanks raw sewage from the works was pumped by a submersible pump fitted with a 20 mm screen to a constant head tank which discharged into a distribution chamber via an adjustable 30° V-notch weir which permitted control of the flow rate. The flow was distributed equally to each tank from the distribution chamber via two adjustable rectan-
233 gular weirs. Prior to entering the tanks, the two flows each passed upwards through a vertically mounted tube fitted with internal baffles which ensured thorough mixing. When amendment of the raw sewage was necessary, the additions were made to this mixing chamber to ensure homogeneity of the settled sewage prior to its entry to the sedimentation tanks. Sample points were located equidistant between each mixing chamber and the inlet to the sedimentation tank (raw sewage), immediately below the effluent outlet (settled sewage) and in separate storage tanks into which the entire contents of the sedimentation tanks were discharged at the end of each experiment (primary sludge). Details regarding the representative sampling of the primary sludge are given elsewhere (Rossin et al., 1983). The sedimentation tanks were operated in parallel, each receiving the same flow but with differing suspended solids concentrations. Thus eight paired experiments were undertaken so that performance at four different influent suspended solids concentrations for each of four different flow rates could be assessed. The flow rates used were 0.055, 0.11, 0.33 and 0.551 s -1, corresponding to 0.5, 1.0, 3.0 and 5.0 DWF. These flow rates were equivalent to surface loadings in the range 6.1-61mday 1 thus, the detention time was 4.6h at 0.5 DWF and 0.5 h at 5 DWF. When suspended solids amendment was required, primary sludge from the sedimentation tanks of the works was collected following a desludging operation and stored in 601 aspirators. The sludge was diluted with primary effluent from the works to form a slurry which was transferred to the sewage flow just after the distribution system and immediately prior to the mixing chamber. In this way, the influent suspended solids concentration could be increased by nominal increments of 150, 300 and 500mg1-1, so that the range of solids concentrations studied was approximately 400-1000 mg l-1. Amendment of the raw sewage with heavy metals was achieved by pumping concentrated standard solutions (unacidified) into the system at the point where the sewage passed over the V-notch weir. These solutions were prepared in such a way that the nominal final concentrations of heavy metals in the raw sewage were 0.5mgl 1 for Zn, 0.1mgl -~ for Co, Cr, Cu, Mn, Ni and Pb, 0.05mg1-1 for Mo and 0.01 mgl -~ for Ag and Tl. These concentrations are typical of those found in mixed industrial/domestic wastewaters. The duration of each experiment was dependent on the flow rate. The sedimentation tanks were filled with raw sewage, the flow was started and solids amendment, where appropriate, was initiated. One retention time was allowed to elapse prior to the addition of heavy metals. After one further retention time, sampling was started and the pilot plant continued in operation for a period equivalent to 10 retention times. Thus, at one dry weather flow (DWF), for example, sampling was continued for 24 h. The sampling frequency was also dependent on the flow rate, and was designed so that flow-proportional composites could be obtained. Each composite was made up from six individual samples. The number of composites corresponded to the number of retention times elapsed during each experiment (i.e. 10 composites).
234 TABLE 1 Analytical data for heavy metal determinations (including data from Sterritt and Lester (1980) and Kempton et al. (1982)) Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Typical conc. in sewage (]~gl 2)
Typical analytical working range (pgl 2)
Relative standard deviation (%)a
Reported detection limit (ttgl-1) b
0.5 4 3 100 500 15 5 100 50 <1 100
1-2O 1 20 1 10 10-100 5--50 1 ~ 100 2 20 2-20 20-200 5-50 1 10 20-200
5 7 15 5 2 8 9 6 7 5 11 3
0.005 0.2 0.003 0.02 0.01 0.02 0.01 0.02 0.1 0.05 0.1 0.001
a Defined as standard deviation divided by mean concentration, calculated from five replicate samples. bInstrument manufacturers' data.
Analytical techniques Suspended solids concentrations (SS) and chemical oxygen demand (COD) were determined by U.K. standard methods (Government of Great Britain, 1977, 1984). For all experiments involving heavy metals, borosilicate ('Pyrex') glass, polytetrafluoroethylene (PTFE) or polypropylene apparatus was used. This was cleaned with a proprietary detergent, rinsed thoroughly and soaked for 24 h in 10% v/v Analar grade nitric acid prior to use. Samples for heavy metal analysis were acidified to 1% (v/v) ~Aristar' grade nitric acid immediately upon collection, except where they were filtered. Filtration was performed on-site and as soon as possible after sample collection, prior to acidification of the filtrates to 1% (v/v) nitric acid. Heavy metals were determined by atomic absorption spectrophotometry. Flame and flameless atomic absorption determinations were made using a Perkin -Elmer model 5000 double beam instrument fitted with standard burner heads and an HGA-500 graphite atomiser respectively. Details of the analytical procedures and sample pretreatments have been reported previously for Cd, Cr, Cu, Ni, Pb, Zn, Ag, Co, Mn and Mo (Sterritt and Lester, 1980) and Bi and T1 (Kempton et al., 1982). Precision data, normal analytical working ranges and reported detection limits for these analytical procedures are shown in Table 1.
235 TABLE 2 The regression of soluble metal on total metal concentrations in raw sewage Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Regression coefficient (b) 0.11 0.09 0.43 0.84 0.91 0.32 0.72 0.68 0.69 0.02 0.50 0.56
Standard deviation (/~gl 1) 1.8 4.0 5.2 4.9 6.6 48 38 150 15 2.1 2.2 90
Range of soluble metal concentration (pgl 1) 0.5-9.2 1.2-4.5 0.5~43 14~8 1.2-400 30-340 20-750 150~830 7.5~400 5-17 6.2 27 200-2000
t-test level of significance b>0
b
0.05 0.01 0.01 0.01 0.001 0.01 0.001 0.10 0.001 0.02 0.01 0.001
0.001 0.001 0.002 NS 0.01 0.001 0.01 NS 0.01 0.001 0.01 0.001
Batch experiments S c r e e n e d and c o m m i n u t e d r a w sewage to be used in b a t c h tests was o b t a i n e d from the Hogsmill V a l l e y W o r k s at a p o i n t just prior to its e n t r y to the p r i m a r y s e d i m e n t a t i o n tanks. It was t r a n s p o r t e d to the l a b o r a t o r y and used for experim e n t a t i o n w i t h i n 12h to p r e v e n t significant d e t e r i o r a t i o n due to microbial activity. Since the r a w sewage e n t e r i n g the Hogsmill w o r k s is almost entirely of domestic origin, it was n e c e s s a r y to add h e a v y metals to it, in o r d e r to i n v e s t i g a t e the effects of metal c o n c e n t r a t i o n and to o b t a i n c o n c e n t r a t i o n s in the influent to the pilot p l a n t typical of mixed i n d u s t r i a l / d o m e s t i c sewage. E l e v a t e d c o n c e n t r a t i o n s of h e a v y metals in the r a w sewage were o b t a i n e d by a d d i n g small aliquots (20-200 pl) of unacidified m e t a l stock solutions to 200 ml samples w h i c h were s u b s e q u e n t l y allowed to equilibrate with mixing for 3 h p r i o r to c o n t i n u i n g with f r a c t i o n a t i o n procedures. The same sewage samples were also amended, w h e r e appropriate, by the s i m u l t a n e o u s addition of prim a r y sludge (2-4 ml) in order to i n c r e a s e the suspended solids c o n c e n t r a t i o n by i n c r e m e n t s of 1 5 0 m g l -~, or by the addition of 1.5-3ml of a c o n c e n t r a t e d s y n t h e t i c sewage ( S t o v e l a n d and Lester, 1980) to i n c r e a s e the soluble COD by i n c r e m e n t s of a p p r o x i m a t e l y 150 mg l- 1.
Metal fractionation H e a v y m e t a l c o n c e n t r a t i o n s were d e t e r m i n e d in the filtrate o b t a i n e d after filtration of a 10ml subsample t h r o u g h a 0.2#m pore size cellulose a c e t a t e m e m b r a n e filter m o u n t e d in a v a c u u m filtration manifold (Amicon Ltd., Stone-
236 house, Gloucs.). Cleaning and pretreatment of the filters was done in a manner similar to that described by Laxen and Chandler (1982). Metals passing the 0.2 pm filters were taken to be soluble (Laxen and Chandler, 1982), although the use of the term 'solubility' in this paper does not imply any thermodynamic significance. RESULTS
Effects of total metal concentration on the proportion of filterable metal A batch test experiment was designed in order to assess the variation in heavy metal solubilities over a range of total concentrations, and to determine whether total metal concentration and filterable metal concentration in raw sewage were related. Six samples of raw sewage (200 ml) were used: the first remained unamended, containing indigenous heavy metal concentrations as a control. To five other samples, specific volumes of stock solutions of heavy metals were added so as to produce five amended sewages with metal concentrations spread over an approximately 10-fold range. Small volumes of stock solutions were added to avoid dilution of the sewage sample and also to maintain a constant pH. The results are presented in Table 2 for the regression of filterable metal concentration on total metal concentration. A t-test was used to evaluate the significance of the data. A regression coefficient not significantly different from zero would be indicative of a constancy of the concentration of soluble metal with increasing added total metal concentration. A regression coefficient not significantly different from unity would indicate that all of the added metal remained in the soluble form. The data in Table 2 shows that, with the exception of Co and Mo, 1 > b > 0 for all metals with a generally high level of significance. The standard deviation quoted refers to the variation in soluble metal concentrations. Thus, a relatively constant fraction of each metal was in the soluble form irrespective of the total concentration. It was concluded, therefore, that in order to obtain the requisite concentration of each heavy metal in raw sewage for experimental purposes, amendment with stock solutions could be used without having a major effect on the fraction of the total metal existing in a soluble form.
The influence of suspended solids and soluble COD concentrations on soluble metal concentrations Batch experiments were performed in order to assess the interaction of suspended solids and soluble COD and their individual and combined effects on the concentrations of metals in the soluble form. The experiments were designed in such a way that the influence of three different suspended concentrations could be examined at each of three different COD concentrations, giving a total number of samples of nine. This approach permitted the determination of any
237 TABLE 3 P e r c e n t a g e solubilities of toxic e l e m e n t s at v a r y i n g solids a n d COD c o n c e n t r a t i o n s for i n d i g e n o u s a n d i n c r e a s e d toxic e l e m e n t c o n c e n t r a t i o n s Metal
COD ( m g l ~)
S u s p e n d e d solids c o n c e n t r a t i o n (rag 1-~) M e t a l s added 475
590
Metals not added 760
475
590
760
Ag
170 330 475
2.5 2.4 2.9
0.8 1.6 2.0
0.9 1.1 0.8
5.8 4.5 4.1
5.5 2.6 4.1
2.8 2.6 2.2
Cd
170 330 475
62.7 68.7 51.9
34.5 37.6 49.0
38.0 32.3 24.6
44.2 76.7 62.9
65.8 54.5 55.6
52.8 45.2 48.6
Co
170 330 475
68.9 71.1 70.5
66.3 58.5 66.0
60.9 64.0 54.9
ND ND ND
ND ND ND
ND ND ND
Cr
170 330 475
59.8 60.8 57.6
32.3 31.6 34.5
21.3 25.0 26.0
4.6 6.3 5.1
6.4 5.7 11.9
9.5 ].5 6.8
Cu
170 330 475
22.5 22.3 21.0
6.4 6.7 13.5
5.2 8.2 6.3
11.2 14.0 16.0
5.8 6.3 7.2
3.4 4.7 5.1
Mn
170 330 475
66.5 71.2 60.7
64.1 64.6 72.2
67.4 70.6 64.1
53.2 57.3 47.4
53.5 52.0 53.6
43.5 49.4 50.0
Mo
170 330 475
66.7 68.0 52.7
62.0 72.4 56.1
58.6 51.7 47.2
ND ND ND
ND ND ND
ND ND ND
Ni
170 330 475
79.2 74.4 67.4
65.2 64.6 75.9
67.5 64.5 62.5
ND ND ND
ND ND ND
ND ND ND
Pb
170 330 475
16.4 11.4 10.4
6.9 8.0 11.0
12.3 7.7 8.0
7.4 18.6 8.3
12.7 6.5 9.6
T1
170 330 475
59.3 52.9 41.9
18.0 15.2 18.0
14.0 15.4 13.3
ND ND ND
ND ND ND
ND ND ND
Zn
170 330 475
21.2 23.6 30.4
22.1 16.2 27.3
26.6 27.2 22.2
50.0 35.3 46.7
83.3 85.0 54.2
29.2 20.0 58.3
6.5 3.4 3.3
238 RAW SEWAGE (sl)
Filtration
FILTERED RAW SEWAGE (S2)
Iteavv metal
add i t i/in
METAL-DOSED RAW SEWAGE (ss)
[ F [I [ rat inn
RAW ~EWAGE FILTRATE ($6) Fig. I. Experimental m e t h o d h e a v y metal removal.
Heavy' metal addition
METAL-DOSED RAW SEWAGE (s3)
I F] I trat ion
REFILTER~:D METAL-DOSED FILTRATE ($4) used to assess the contribution of adsorption a n d precipitation to
possible interactive effects between the two parameters. Equal quantities of metal stock solutions were added to each sewage sample prior to equilibration for 3 h followed by filtration and determination of the soluble metal. Similar experiments were performed using the indigenous metal concentrations (i.e. those in Table 2) where possible. Table 3 shows the percentage solubilities of metals in each sample at varying solids and COD concentrations for added and indigenous metal concentrations. No results are presented for Bi, as every filtrate contained less than 0.0005 mg l- 1. From the data presented in Table 3 several metals appeared to be influenced by solids concentrations, a decrease in filterable metal corresponding to an increase in solids concentration. These metals were Ag, Cd, Cr, Cu and T1. The soluble metal fraction did not, however, appear to be markedly affected by COD, although in the case of copper an increase in COD was accompanied by increasing filterable metal concentration. These data suggested that soluble organic ligands may play a minor role in determining the proportion of metal in the soluble form in raw sewage.
239 TABLE 4 Potential removals of heavy metals as influenced by adsorption and precipitation in raw sewage Metal
Fraction of total metal (%) Precipitation calculated from $3 - $4 x I00 $3
Total Adsorption calculated column 3 from minus column 2 $5 - S6 x 100 $5
Total removal from Eqn (1)
Adsorption column 5 minus column 2
89 89 69 41 23 81 49 4 22 94 43 73
76 89 88 41 12 100 0 4 22 93 43 100
65 37 88 41 10 89 0 2 22 22 43 58
-
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
11 52 0 0 2 11 4 2 0 71 0 43
-
78 37 69 41 21 70 45 2 22 24 43 31
Adsorption and Precipitation Samples of r a w sewage were used, either filtered or unfiltered, to assess the e x t e n t of a d s o r p t i o n of metals by the solids and p r e c i p i t a t i o n in a solids-free sample, in o r d e r to d i s t i n g u i s h b e t w e e n two m a j o r r e m o v a l m e c h a n i s m s for e a c h metal. By e m p l o y i n g sewage samples with a n d w i t h o u t h e a v y m e t a l additions and filtering b o t h before and after s u c h additions, the p o t e n t i a l r e m o v a l s by p r e c i p i t a t i o n a n d by a s s o c i a t i o n with solids were investigated. The e x p e r i m e n t a l scheme used is r e p r e s e n t e d in Fig. 1, and the results are s h o w n in Table 4. F r o m the samples d e n o t e d S1 and $2 in Fig. 1, the soluble metal c o n c e n t r a t i o n and h e n c e m a x i m u m p o t e n t i a l r e m o v a l of metals was calculated. H o w e v e r , to enable a more direct c o m p a r i s o n b e t w e e n different metals t h e i r c o n c e n t r a t i o n s were increased to similar levels by the addition of s t o c k solutions and h e n c e samples $5 and $4 were used to c a l c u l a t e the p o t e n t i a l r e m o v a l s at these elevated c o n c e n t r a t i o n s . Samples $3 and $4 were used to estimate p o t e n t i a l r e m o v a l due to p r e c i p i t a t i o n m e c h a n i s m s alone. Thus, by s u b t r a c t i n g r e m o v a l due to p r e c i p i t a t i o n from t o t a l removal, a figure was o b t a i n e d w h i c h could be e q u a t e d to r e m o v a l by a d s o r p t i o n o n t o the solid phase. F u r t h e r c a l c u l a t i o n s were performed to d e t e r m i n e the r e m o v a l percent a g e of added metals from the following equation: R e m o v a l (%)
=
($5 - S1) - ($6 - $2) x 100 ($5 - S1)
(1)
240 TABLE 5 Influence of suspended solids c o n c e n t r a t i o n s and hydraulic loading on solids removal (%) Hydraulic loading (DWF)
0.5 1.0 3.0 5.0 Correlation coefficient a Level of significance
Influent suspended solids c o n c e n t r a t i o n (mgl 1) 405
553
733
888
70 62 47 56 - 0.67 NS
74 63 69 55 0.05 NS
79 64 68 60 - 0.71 NS
77 65 65 66 - 0.55 NS
Mean
Correlation coefficient a
Level of significance
75 64 62 59
0.86 1.00 0.64 0.91
0.10 0.001 NS 0.05
a Based on arcsin t r a n s f o r m a t i o n of removal data.
This gave the total removal of added metals and hence removal due to adsorption could be calculated as above. Sample $2 gave inconsistent results for Cd, Cu and Zn; these were probably due to contamination as a result of their initially very low concentrations and values from sample $6 were substituted. Although the estimated potential removal by adsorption varied according to the method of calculation, the relative proportions of removal by precipitation and adsorption were consistent, except in the case of Mn and Zn. Therefore, it was possible to predict which metals would be influenced to the greatest extent by adsorption or precipitation and whether the solid or soluble phase would be the dominant factor in effecting removal. The extent to which this potential removal actually occurs will depend, however, on the settleability of the particulates formed. The behaviour of Bi and Pb appeared to be primarily dominated by precipitation; Ag, Cd, Co, Cu, Cr, Mn, Ni and T1 removals were controlled by adsorption and Zn and Mo removals were influenced by both factors.
Influence of solids and surface loading on solids removal efficiency At each of four hydraulic loadings and four influent suspended solids concentrations, the suspended solids removal efficiencies were calculated. These are shown in Table 5. The arcsin transformation was used to normalise the percentage solids removal data, prior to calculation of the correlations betJ ween solids removal and influent suspended solids concentration and hydraulic loading. It is apparent that the solids removal efficiencies increased with an increasing influent solids concentration and tended to decrease with an increasing hydraulic loading. However, none of the correlation coefficients for the latter effect were significant, indicating that the major influence is influent suspended solids concentration.
241 TABLE 6 M e t a l solids a n d copper c o n c e n t r a t i o n s in i n f l u e n t d i s c r e t e a n d c o m p o s i t e s a m p l e s C o n c e n t r a t i o n (mg1-1) Discrete sample values Suspended solids
Mean
Composite
534
524
226
498
352
320
416
392
198 318 388 268 144 510 286 302
338 348
350 334 144 88 398 664 232 1174
326 220 486 194 352 642 316 274
292 346 208 552 606 526 462 464
162 412 912 98 790 426 540 394
278 330 356 258 415 564 365 509
276 344 428 302 436 578 370 556
388
409
0.156 0.112 0.232 0.084 0.128 0.455 0.255 0.310 0.220
0.255 0.157 0.195 0.167 0.193 0.250 0.300 0.243 0.292
0.256 0.156 0.206 0.176 0.210 0.252 0.270 0.252 0.280
Overall m e a n
0.228
0.229
346 344 618 352 444
Overall mean Copper 0.118 0.198 0.228 0.172 0.168 0.290 0.220 0.260
0.306 0.162 0.210 0.138 0.244 0.172 0.320 0.225 0.255
0.306 0.180 0.198 0.120 0.116 0.186 0.320 0.145 0.355
0.276 0.192 0.146 0.280 0.180 0.202 0.345 0.220 0.400
0.230 0.176 0.188 0.154 0.320 0.316 0.300 0.340 0.260
Influence of solids concentrations and hydraulic loading on heavy metal removal Factors which may influence the removal efficiencies of heavy metals during primary sedimentation were also investigated during operation of the pilot plant at four different influent solids concentrations at each of four different hydraulic loadings. During these experiments it was found necessary to prepare composite samples for analysis, and so the validity of doing this was assessed by comparing results from both individual discrete samples and composite samples. The outcome of these experiments for suspended solids and for copper determinations is shown in Table 6. The final two columns in Table 6 were compared using a t-test, and were not found to be significantly different at the 5% level of significance. The practise of compositing samples was therefore considered acceptable. Removal efficiencies as percentages have been summarised in Table 7 for the 12 metals studied. Throughout these experiments mean removal efficiencies varied from 4 to 54%. It is apparent from Table 7 that both solids concentration and flow rate exerted an influence on the removals of the heavy metals. Generally, increased
242 TABLE 7
The influence of influent suspended solids concentration and hydraulic loading on heavy metal removal (mean removal efficiency (%)) Metal
Suspended solids concentrations
Hydraulic loading (DWF)
(mgl-')
Ag Bi Cd Co Cr Cu
Mn Mo Ni Pb
T1 Zn
405
553
733
888
32 10 17 15 16 31 22 4 23 36 23 32
41 19 25 24 24 40 27 11 23 42 27 37
37 8 25 20 14 38 25 12 19 38 35 33
52 19 24 34 28 49 30 13 36 52 54 52
0.5
1.0
3.0
5.0
54 16 24 30 29 54 36 13 30 52 42 46
44 17 26 28 23 23 21 14 22 40 38 36
35 7 17 20 28 43 23 6 15 42 31 39
29 16 24 14 11 38 23 7 34 34 29 32
removals were observed with an increasing solids loading and decreased removals were observed for increasing flow rates. Correlation coefficients were calculated for heavy metal removal efficiencies against influent solids concentrations, for each flowrate, again using the arcsin transformation of the heavy metal removal efficiency data. These results are summarised in Table 8. With the exception of Bi, Cr and Cu the correlation between metal removal and influent suspended solids concentration at 1 DWF was significant with P < 0.05. Mass balances Mass balances were calculated for each metal during each experimental run of the pilot plant in order to indicate the reliability of the nature of sampling, particularly in the case of the sludge suspension, which was sampled only once at the end of each experiment. Mass balances were calculated from the following equation: Mass balance
=
[ VI " M I ]
WE'ME] + [½"Ms]
×
100(%)
(2)
where V1 = total influent volume (1), VE = total effluent volume (1), Vs = sludge suspension volume (1), M[ = heavy metal concentration in influent (rag l- ~), M E = heavy metal concentration in effluent (mg l-~), Ms = heavy metal concentration in sludge suspension (rag 1 1). The results of the mass balance calculations are presented in Table 9. Generally the mass balances for each metal at each experiment were accept-
243 TABLE 8 Correlations a between heavy metal removal and influent suspended solids concentrations at different hydraulic loadings Metal
Ag Bi Cd Co Cr Cu Mn Mo Ni Pb T1 Zn
Hydraulic loading (DWF) 0.5
1.0
3.0
0.82 0.23 - 0.14 0.07 0.18 0.81 0.14 0.96* - 0.01 0.26 0.71 0.22
0.88* - 0.29 0.97* 0.97* 0.65 0.65 0.92* 0.56 0.99* 0.90* 0.90* 0.98*
0.75 0.25 0.84 0.80 0.75 0.79 0.81 0.26 0.80 0.59 0.84 0.82
5.0 0.45 0.93* 0.05 0.95* 0.74 0.98* 0.79 0.59 0.18 0.74 0.69 0.85
Based on arcsin transformation. * Statistically significant at p = 0.05. able. By c a l c u l a t i n g m e a n v a l u e s from the mass b a l a n c e s d a t a for each metal and for each e x p e r i m e n t a l r u n an overall i n t e r p r e t a t i o n of individual mass b a l a n c e s was obtained. Thus, m e a n mass b a l a n c e s for each metal d u r i n g all e x p e r i m e n t a l r u n s gave v a l u e s from 93 to 106% and for all metals e a c h experim e n t a l r u n gave v a l u e s from 84 to 148% with a c o m m o n m e a n for all the d a t a of 100%. These results i n d i c a t e t h a t the a n a l y t i c a l m e t h o d o l o g y for every metal studied was reliable, and t h a t d e v i a t i o n s from the 100% expected v a l u e were l a r g e l y due to the effects of process o p e r a t i o n . In p a r t i c u l a r , the h o m o g e n e i t y of the sludge s u s p e n s i o n was critical in p r o d u c i n g good mass balances. No distinct t r e n d s were a p p a r e n t for m e a n mass b a l a n c e s in terms of solids loadings at each flow rate. DISCUSSION
The r e m o v a l of h e a v y metals d u r i n g w a s t e w a t e r t r e a t m e n t is an advant a g e o u s c o n s e q u e n c e of the existence of settleable particles in the influent and c o n v e r s i o n of soluble and non-settleable to settleable forms d u r i n g t r e a t m e n t . Thus, the p h y s i c a l r e m o v a l of settleable forms of h e a v y metals is largely influenced by the solids removal. D u r i n g the c o u r s e of the p r i m a r y sedimentation pilot p l a n t experiments, a c c e p t a b l e r e m o v a l s of suspended solids from the raw sewage influent were a c h i e v e d a n d it was a p p a r e n t t h a t the s e d i m e n t a t i o n t a n k s were p e r f o r m i n g s a t i s f a c t o r i l y in c o m p a r i s o n with the predictions of t h r e e t h e o r e t i c a l models. It is evident t h a t the a v e r a g e r e m o v a l efficiencies r e c o r d e d in these e x p e r i m e n t s c o m p a r e well with the v a l u e s predicted for
244 TABLE 9 M a s s b a l a n c e s for h e a v y m e t a l s d u r i n g t h e p r i m a r y s e d i m e n t a t i o n s t u d y Hydraulic loading (DWF)
Suspended solids ( m g l 1)
Ag
Bi
Cd
Co
Cr
Cu
Mn
Mo
Ni
Pb
T1
Zn
Mean
0.5
405 553 733 888
114 174 135 110
81 160 85 98
97 165 91 89
116 161 98 111
122 164 82 114
108 144 115 116
114 130 99 93
103 104 111 121
151 116 95 150
106 174 82 116
97 127 86 95
104 159 88 129
109 148 97 112
1.0
405 553 733 888
95 85 83 82
111 58 90 81
99 98 105 100
97 97 101 98
106 98 92 80
115 91 94 83
107 99 85 74
99 104 111 103
100 91 99 98
101 73 74 61
115 102 103 86
100 67 95 83
104 89 94 86
3.0
405 553 733 888
83 87 89 130
96 86 86 109
96 91 90 109
93 90 93 128
86 102 93 103
81 83 75 148
76 80 72 111
87 102 105 89
85 82 93 95
72 77 73 142
89 73 71 130
103 95 85 147
87 87 85 120
5.0
405 553 733 888
76 71 75 119
99 97 102 105
106 98 99 95
77 89 100 101
99 80 99 100
94 75 89 114
86 79 78 98
94 102 95 101
125
85 69 66 112
105 77 97 113
84 88 101 122
94 84 92 107
100
96
102
103
101
102
93
102
93
98
103
100
Mean
M a s s b a l a n c e s (%)
109
106
full-scale tanks by the three theoretical models, the results lying within the range of the values at lower hydraulic loadings derived by Smith (1969) and Tebbutt and Christoulas (1975) and the perhaps somewhat optimistic values predicted for higher hydraulic loadings which have been discussed previously (Anderson, 1981). Similarly, the suspended solids removal efficiencies obtained at varying influent suspended solids loading, which was a major variable studied during these experiments, compare favourably with those derived previously (Tebbutt and Christoulas, 1975; Anderson, 1981) as do the values obtained for non-settleable solids. Both solids loading and hydraulic loading appear to influence the removal efficiencies of solids. Optimum removals were generally obtained for higher solids loadings and for the lowest flowrate. It would appear, however, that a decline in solids removal occurred at the upper extreme of solids loading in some cases. At flows equivalent to 0.5 and 3 DWF, the maximum solids removal occurred at an intermediate solids loading, indicating that the plant may have been overloaded. Therefore, in practice, the operation of primary sedimentation in terms of solids and hydraulic loading was critical, and clearly the greater precision in controlling these parameters in keeping with the design limits would give optimum performance in terms of solids removal. The heavy metal mass balance calculations for the primary sedimentation studies indicated the greatest variation in observed removal efficiencies to be
245 due to the effects of operational parameters rather than analytical or sampling error. Average removals were 41% Ag, 14% Bi, 23% Cd, 23% Co, 20% Cr, 39% Cu, 26% Mn, 10% Mo, 25% Ni, 42% Pb, 35% T1, and 38% Zn. These results generally agree with reported values for removal efficiencies during primary sedimentation (Oliver and Cosgrove, 1974; Rossin et al., 1983), although the variation in removal efficiencies has resulted in relatively poor mean values in some cases. The variation in influent toxic element concentrations was minimised by continuous addition of stock solutions. However, some variation was unavoidable since raw sewage and primary sludge quality was dependent on external factors. The sampling system adopted after comparing discrete with composite sampling was intended to minimise any external factors so enabling experiments to be performed on a comparative basis. The fairly wide variations in heavy metal removal efficiencies suggested that the operating parameters for primary sedimentation can significantly influence heavy metal removal. These experimental results have demonstrated the influence of both solids loading and flowrate on heavy metal removal efficiencies. The purely physical processes of settling particulate forms of heavy metals would be expected to remove very little of the soluble forms of heavy metals (Oliver and Cosgrove, 1974). Therefore, as expected, the removals of heavy metals were related to solids removal. There were some significant correlations between heavy metals removals and suspended solids concentrations. Since solids removals were found to be influenced to a greater extent by influent solids concentraion than by overflow rate, it is not suprising that heavy metal removal efficiencies were influenced in this way. This suggests t h a t the distributions of toxic elements and particulate matter were similar if not coincident. This possibility is investigated in a subsequent paper (Kempton et al., 1987). CONCLUSIONS Among the factors which would be expected to influence heavy metal solubility in raw sewage, the suspended solids concentration had the greatest effect on Ag, Cd, Cr, Cu and T1, with reductions in metal solubility of 50% or more accompanying increases in suspended solids of up to approximately 50%. The percentage solubilities of these metals were also largely unaffected by increases in added metal concentrations. Metal solubility was not significantly influenced by soluble COD, indicating that the solid phase was the dominant controlling factor. Much of the metal in the solid phase was adsorbed onto the suspended solids. The formation of precipitates was significant only in the case of Bi, Pb and Zn. The pilot scale primary sedimentation plant performed according to predictions of full scale plants. Solids removal efficiency was in direct proportion to solids loading, whereas increasing the hydraulic loading caused marked reductions in solids removal. These factors were reflected in the heavy metal removal efficiencies obtained, the closest correlations between metal removal and solids loading were obtained for several elements, particularly those of low solubility.
246 ACKNOWLEDGEMENTS The authors acknowledge the generous cooperation of Mr E Eves, Manager o f t h e H o g s m i l l V a l l e y W a t e r P o l l u t i o n C o n t r o l W o r k s . O n e o f u s ( S K ) is grateful to the Science and Engineering Research Council for the award of a postgraduate studentship. REFERENCES Anderson, J.A., 1981. Primary sedimentation of sewage. Water Pollut. Control, 3: 413-420. Barth, E.F., M.B. Ettinger, B.V. Salotto and G.N. McDermott, 1965. Summary report on the effects of heavy metals on the biological treatment processes. J. Water Pollut. Control Fed., 37: 86-96. Feiler, H.D., A.S. Vernick and P.J. Stotch, 1979. Fate of priority pollutants in POTW's. Municipal Sludge Management, Proc. 8th National Conf., Miami, Information Transfer Inc., Silver Springs, MD, pp. 72-81. Government of Great Britain, 1978. Chemical Oxygen Demand (Dichromate Value) of Polluted and Waste Waters 1977. Methods for the Examination of Waters and Associated Materials. HMSO, London, 14 pp. Government of Great Britain 1984. Suspended Settleable and Total Dissolved Solids in Waters and Effiuents 1980. Methods for the Examination of Waters and Associated Materials. HMSO, London, 27 pp. Kempton, S., R.M. Sterritt and J.N. Lester, 1982. Atomic absorption spectrophotometric determination of antimony arsenic, bismuth, tellurium, thallium and vanadium in sewage sludge. Talanta, 29: 675~78. Kempton, S., R.M. Sterritt and J.N. Lester, 1987. Heavy metal removal in primary sedimentation II. The influence of metal speciation and particle size distribution. Sci. Total Environ., 63: 247 258. Laxen, D.P.H. and I.M. Chandler, 1982. Comparison of filtration techniques for size distribution in fresh waters. Anal. Chem., 54: 1350-1355. Lester, J.N., 1983. Significance and behaviour of heavy metals in waste water treatment processes. I. Sewage treatment and effiuent discharge. Sci. Total Environ., 30: 1-44. Lester, J.N., R.M. Harrison and R. Perry, 1979. The balance of heavy metals through a sewage treatment works. I. Lead, cadmium and copper. Sci. Total Environ., 12: 13-23. Oliver, B.G. and E.G. Cosgrove, 1974. The efficiency of heavy metal removal by a conventional activated sludge treatment plant. Water Res., 8: 86~874. Rossin, A.C., R.M. Sterritt and J.N. Lester, 1982. The influence of process parameters on the removal of heavy metals in activated sludge. Water Air, Soil Pollut. Rossin, A.C., R.M. Sterritt and J.N. Lester, 1983. The influence of flow conditions on the removal of heavy metals in the primary sedimentation process. Water, Air, Soil Pollut., 19: 105-121. Smith, R., 1969. Preliminary design of wastewater treatment system. J. Sanit. Eng. Div., Am. Soc. Civ. Eng., 95: 117. Sterritt, R.M. and J.N. Lester, 1980. Atomic absorption spectrophotometric analysis of the metal content of waste water samples. Environ. Technol. Lett., 1: 402-417. Stoveland, S. and J.N. Lester, 1980. A study of the factors which influence metal removal in the activated sludge process. Sci. Total Environ., 16:37 54. Stoveland, S., M. Astruc, J.N. Lester and R. Perry, 1979. The balance of heavy metals through a sewage treatment works. II. Chromium, nickel and zinc. Sci. Total Environ., 12: 25-34. Tebbutt, T.H.Y. and D.G. Christoulas, 1975. Performance relationships for primary sedimentation. Water Res. 9: 347-356.