Nitrification and denitrification of tannery wastewaters

Wat. Res. Vol. 25, No. II, pp. 1351-1356, 1991 Printed in Great Britain.All rightsreserved

0043-1354/91 $3.00+0.00 Copyright © 1991PergamonPressplc

NITRIFICATION AND DENITRIFICATION OF TANNERY WASTEWATERS L. SZPYRKOWICZl, S. RIGONI-STERN2@ and F. ZILIOGRANDIl * ~ 1Department of Environmental Sciences, Venice University, Dorsoduro 2137, 30123 Venice and 2Tecno-Bio srl, Cornedo Vicentino, 36073 Vicenza, Italy (First received November 1990; accepted in revised form April 1991) Al~traet--In this paper the studies on the dissimilative removal of nitrogen by biological means from wastewaters consisting of up to 90% chrome tannery and 10% domestic sewages are reported. Experiments were carried out over a 6 month period in a pilot plant of the modified Ludzack-Ettinger configuration. The feasibility of the biological nitrogen removal from this type of wastewaters without a preliminary chemical-physical phase or an external carbon source for denitrification was proved. The COD utilization coefficientwas 12.5 mg COD for 1 mg of denitrified N. No inhibition of the process was induced by Cr or by S2- present in the raw wastewaters. A negative effect on the denitrification rate resulted from a high ratio between the quantity of oxygen returned with the mixed liquor and the inlet COD. Key words--tannery wastewaters, biological treatment, nitrification, denitrification, nitrogen respiration coefficient, single sludge process, Cr and S2- inhibition

INTRODUCTION In the conventional systems the quantity of nitrogen eliminated with wasted sludge is less than 0.025 mg N/mg COD applied [for sludge ages greater than 10 days and the sludge N content of 10% (Ekama and Marais, 1984]. For wastewaters with a TKN/ COD ratio exceeding this value additional nitrogen removal can be obtained by a biological nitrificationdenitrification. Several factors can influence the kinetics of nitrification: NH 4+ concentration itself, dissolved oxygen (DO) concentration and the presence of inhibitory substances being the most important ones. The Haldane equation is normally applied for the description of nitrification when the process is somehow inhibited (Rozich and Castens, 1986; Keenan et al., 1979), whereas a zero order kinetics is used if NH 4+ is present in high concentration (Sutton et al., 1981). Denitrification is generally well described by zero order kinetics (Ekama and Marais, 1984; Abufayed and Shroeder, 1986; Vismara, 1982). To meet the discharge standards for nitrogen, wastewaters produced by chrome tanneries must undergo processes of nitrification and denitrification since they contain high concentrations of nitrogen compounds. A high content of organic matter coupled with the presence of biological inhibitors as Cr and S2- induces using a chemical-physical (C/P) pretreatment, in a traditional treatment scheme of these wastewaters (e.g. Bitkover et al., 1980; Pastore et al., 1984; Ghimenti and Botrini, 1988). In a C/P

*Author to whom all correspondence should be addressed.

phase a part of the organic load is eliminated along with Cr and S2-. Consequently an external carbon source normally is essential to subsequent denitrification due to a subtraction of organic substances from the wastewaters. This kind of treatment is very costly and often troublesome because of the high quantity of sludge produced in a C/P step (due to addition of chemicals) and of the difficulty in its handling and wasting. These were the problems to be solved in a full scale traditional plant (Szpyrkowicz et al., 199 l). The objective of this study was to verify the possibility of a biological nitrification--denitrification of tannery wastewaters in a single sludge system avoiding the C/P phase. A rough evaluation of the kinetic constants of N removal from this type of wastewaters was subsequently done. EXPERIMENTAL

The scheme of the pilot plant was chosen considering that a TKN/COD ratio for raw wastewaters exceeds 0.10 mg N/rag COD. The modified Ludzack-Ettinger configuration is indicated in this case as the most suitable one (Ekama and Marais, 1984). The pilot plant (Fig. 1) consisted of a denitrification unit of 5501., equipped with a vertical stirrer and an aerobic activated sludge unit of 1000I. volume with ceramic candles for aeration. The volumes of the anoxic and aerobic units were established using the parameters of the full scale plant (the rates of N removal were assumed equal to domestic sewage: 0.09 g TKN/g MLVSS x day for nitrification and 0.16 g N-NO3/g MLVSS x day for denitrification), in order to use the results of the study to upgrade it. For flow rates of 25 l/h, the nominal hydraulic residence time was consequently 22 and 40 h, respectively for anoxic and aerobic basins. The mixed liquor was recycled at the head of the denitrification basin by a peristaltic pump. The effluent from the

1351

1352

L. SZPYRKOWICZet al. Biological oxidation

Biological denitrification

Sedimentation

1

SLudge recycle

Effluent

Air

Mixed Liquor recycle

Infkuent

(~

Biological sludge to wa st.e

Fig. 1. Treatment scheme of a pilot plant. aerobic basin was sedimented and the separated sludge at the rate of 100-150% the influent flow was recycled in the denitrification unit by an air-lift pump. The feed consisted of a mixture of discharges from 30 tanneries located in the area of the full scale plant. Tanneries discharge the sedimented and homogenized wastewaters into a consortium collecting system which also receives the domestic sewage from the same area. Domestic sewage contributes to approx. 11% of the total flow, corresponding to 2.5% of the organic load, expressed as COD. Wastewater composition over the 6 months testing period was the significant factor characterizing the 4 trial runs. Liming waters in high percentage were found in runs 2 and 4; their concentration was medium in the first run and low in the third one. The analyses of the raw wastewaters: COD, BODs, TKN (total Kjeldahl nitrogen), NH 4+, Cr tot and TSS were carried out on 24 h time-composite samples; pH, rH and Sz-, were determined as a mean of 4 or more grab samples data. The analyses were conducted according to Standard Methods (APHA et aL, 1989). COD was determined on diluted samples to avoid interferences of C1- ions (> 2000 rag/l). NO 3- and NO 2- were not analysed being practically absent. Table 1 gives the mean values of the influent parameters (further details are reported elsewhere (Szpyrkowicz et aL, 1991). The pilot plant was seeded with the sludge from the full scale treatment plant. Tests were conducted under a pseudo steady-state conditions. Sludge age was maintained at 22 days. The recycling rate of mixed liquor, calculated according to Panzer (Panzer et al., 1981; Panzer, 1982) was R = 10 for the runs 1, 3 and 4 and R = 14 for the second run. The wastewater's mean temperature varied between 20 and 22°C during the 4 runs. Table 2 reports the operational conditions of the plant for the 4 runs and the mean values of MLSS (mixed liquor suspended solids), MLVSS (mixed liquor volatile suspended solids), SVI (sludge volume index) and DO (dissolved oxygen). Both the sludge concentration and the microbio-

logical composition in the denitrification and nitrification units can be considered equal, since the recycling rates were very high. RESULTS AND DISCUSSION

Figure 2 depicts the influent and effluent C O D and N vs time for different runs. Very large fluctuations of the influent C O D are noticeable while T K N concentrations remain relatively constant. Table 3 reports the mean effluent parameters of the 4 runs. The processes of N and C O D removal were not inhibited by a high concentration of N H 4+ nor by the presence of Cr and S = (see Figs 3 and 4), though mean influent concentrations of Cr and S = throughout the 4 runs were considerably high: 32.4 and 49.1 mg/l, respectively. It is presumed that Cr was present mainly in an insoluble form (hydroxide) and was eliminated along with the wasted sludge (low values in the effluent and a high correlation between Cri, and TSSi~ confirm it (Crin -- 9.56 + 0.021 x TSS, r = 0.95). More than 99% of the incoming Cr was removed. Lack of data regarding the anoxic effluent make it virtually impossible to calculate the precise C O D removals separately for anoxic and aerobic basins, though certainly the most proper approach. It is assumed here that all C O D was removed in the anoxic basin. This hypothesis does not introduce a substantial error as in a single-sludge process operating with T K N / C O D ratios as in this study the difference between anoxic and aerobic effluent C O D Table 2. Pilot plant operational parameters (mean values)

Table I. Mean characteristics of the influent wastewater (concentrations in rag/I, except for pH and rH) Parameter

Run I

Run 2

Run 3

Run 4

pH rH COD BOD 5 N-TKN N-NH4 Cry1 S 2TSS

7.3 8.35 2530 1105 371 222 34.2 44 564

7.2 7.83 2766 1040 415 212 32.0 56 937

7.2 8.95 2345 955 367 193 32.0 51 840

7.2 9.22 2744 1021 367 169 32.1 45 1042

Parameter Test duration (days) Flow rate, Q (I/h) MLSS (g/l) MLVSS (g/I) Sludge recycle MLSS recycle, R COD overall load (kg/m 3 × day) SVI (ml/g) 02 aer (rag/l) O2anx (rag/I)

Run I

Run 2

Run 3

Run 4

29 25 3.3 2.5 IQ 10Q 0.98

24 25 3.1 2.3 1Q 14Q 1.07

22 25 3.6 2.7 1Q IOQ 0.92

30 28 4.0 3.0 1.5 Q IOQ 1.19

240 4.7 0.12

316 4.1 0.07

228 4. I 0.06

228 4.3 0.06

Biological treatment of tannery wastewaters Table 3. Mean characteristics of effluent wastewaters (rag/l) Parameter Run 1 Run 2 Run 3 Run 4 COD 146 124 116 134 BODs 22.0 7.6 9.6 6.2 N-NO2 1.39 0.20 0.20 0.17 N-NO3 13.79 12.9 13.9 9.9 N-NH4 7.1 2.3 5.3 3.7 Cr3+ 0.35 0.22 0.22 0.23 is quantitatively insignificant and in the magnitude o f the analytical error (McClintock et al., 1988; A r g a m a n and Brenner, 1986).

1353

During all the runs BOD5 was nearly completely eliminated. Since a certain a m o u n t o f nitrogen still was present in the effluent (about 10 rag/l)---mainly in an oxidized f o r m - - f u r t h e r denitrification could be possible if the plant was not underioaded for C O D . However, this was not the objective o f the present study since the pilot plant effluent met the discharge limits. Precluding any precise kinetic consideration, the T K N / C O D ratio in the influent was practically equal to the ratio between the N and C O D removal rates.

I-

4000

+

+

:5000

/

.I-

+ l

+

2000

+

COOin TKNin

1000

A

E 140

120

I O0

80

60

40

o

COOou~

+

(NH+ + NO2+ NO3)out

t-

A

20

+ -r "1" I't" '~.+

0 20

I 60

+ "If 100 Time

I

I

140

180

(doys)

Fig. 2. Time variations of influent and effluent COD and nitrogen.

L. SZPYRKOWICZ et al.

1354

Table 4. Pilot plant treatment efliciencies Removal efficiencies

Run Run Run Run

1 2 3 4

COD t/

BOD 5 r/

NH4~ r/

N,ot rt

Cr r/

0.94 0.95 0.95 0.95

0.98 0.99 0.99 0.99

0.97 0.97 0.97 0.98

0.93 0.96 0.95 0.96

0.99 0.99 0.99 0.99

Process rates ................... DNR Yt }. 0.14 0.17 0.13 0.13

1.04 1.22 0.94 1.09

NIT

1.00 1.16 0.88 1.03

0.087 0.107 0.080 0.081

DNR = Nitrogen removal rate by denitrification (mg N/mg MLVSS x day). Yt = Total COD removal rate in denitrification basin (mg COD/rag MLVSS x day). Y, = COD removal rate by denitrification only (mg COD/mg MLVSS x day). NIT = Nitrification rate (mg N/mg MLVSS x day).

Table 4 reports the efficiencies (r/) and the rates (mg/mg MLVSS x day) of COD and N removal during separate runs. The calculation of the N balance was done assuming the requirements for sludge production equal to 0.016mg N/mg COD (Ekama and Marias, 1984) and considering that the remaining part of incoming T K N was eliminated by nitrification-denitrification. Zero order kinetics were employed to calculate the nitrification since no inhibition of the process was observed. The obtained rates varied between 0.08 and 0.11 mg N/mg MLVSS x day and are comparable with values typical for a domestic sewage. Zero order kinetics were also applied to calculate denitrification (nitrate removal was independent of the nitrate concentration). The highest mean apparent denitrification rate (the term "apparent" is used since in the completely mixed systems the reactions with the soluble and particulated COD occur simultaneously) was equal to 0.17 mg N/mg MLVSS x day and was achieved in the second run, characterized by the highest influent T K N and COD. Calculating the quantity of DO which entered the denitrification unit with the mixed liquor and assum-

ing the oxygen consumption for substrate utilization equal to 0.33 mg O2/mg COD (Ekama and Marais, 1984), the quantity of COD used in denitrification for oxygen respiration (Yo) was defined. It resulted that approx. 6% of the incoming COD was eliminated with 02 as an electron acceptor instead of the NO~(see Table 4). COD used only for denitrification (Y,) was calculated as the difference between the total COD removed (Yt) and the Yo value. The relationship between nitrate reduction and COD involved in denitrification can be expressed in a way analogous to the oxygen utilization in an aerobic system (Argaman and Brenner, 1986): D N R = an, x Yn + b., where D N R = denitrification rate (rag N/rag MLVSS x day) an, = nitrates utilization coefficient (g NO3-N/g COD) bn,=nitrates endogenous respiration rate (g NO3-N/g VSS × day).

!.6 +

14

+

>, o

10

x 03 03

+ 0.8

~"

+

+

06

+

COD r e m o v a l

o

N removal

rote

rete

0.4

j,----.o 02

5

2.5

Cr s? c o n c e n t r o t i o n

in o denitrilicotion

I

I

5.5

4.5

bosin

(mg/L)

Fig. 3. Influence o f Cr c o n c e n t r a t i o n in a denitrification basin o n C O D a n d N r e m o v a l rates.

Biological treatment of tannery wastewaters

1355

t.8 1.6 ÷ 1.4 o

"o x

+T.+ [ \ / +

1.2

1,0

0,8 E 0.6

+

COD removal, rate

0

N

removal rate

0,4

0.2 ,?r-o

o o

i

-

I

I

I

I

I

I

2

3

4

5

6

7

S2-concentrotion

in a d e n i t r i f i c a t i o n

basin

( mg / I. )

Fig. 4. Influence of S2- concentration in a denitriflcation basin on COD and N removal rates. Plotting Y. vs D N R (Fig. 5), as, = 0.08 g NO3-N/g COD and b., = 0.05g NO3-N/g VSS x day were obtained, It results that up to 12.5 mg of COD are required to reduce 1 mg NO3-N. Though the DO concentration in the denitrification basin (0-0.1 rag/l) was far below the inhibiting values, oxygen r~ycled with the mixed liquor did influence the rate of the denitdfication, likely by consumption of the readily biodegradable organic matter. The denitrification rate diminished with an increase in the

ratio between the 02 recycled into the denitrification and the incoming COD, according to: D N R = 0.18 - 2.61 x(~O:

returned/COD=)

(r=0.65).

The denitrification rate was not inituenced by the mass of the recycled 02 itself as an independent variable.

0.22 --

O.20 A :=.,

0

"o x (/) (./)

0.18

0.16

:E E vE nZ O

0.14 - -

0.12 L D N R - 0.05 + 0 . 0 8 Yn 0.10

Q08

Q06 0.6

I

I

I

I

I

1.0

1.2

1.4

1.6

(rngCOD/rng

MLVSS

xdoy)

Fig. 5. Determination of nitrate consumption coefficients: Y, = COD removal rate in a denitrification basin (rag COD/rag MLVSS x day); DNR -- denitrification rate (mg N/mg MLVSS x day).

L. SZPYRKOWICZet al.

1356 CONCLUSIONS

The results of this study demonstrated the possiblity to treat biologically sedimented wastewaters from chrome tanneries without a C / P pretreatment. The use of an external carbon source for denitrification was unnecessary. In terms of the final effluent quality and of N and C O D removal, good efficiencies were achieved. The rate coefficients for nitrification and for denitrification were comparable to the values for domestic sewage. Wastewater composition variances related to differences in tanning processes caused no substantial change in the treatment efficiency. No inhibition of biological processes by S2- and Cr was observed. The results showed that in single-sludge systems the denitrification rate is influenced by the ratio between the returned oxygen and the C O D of inlet wastewaters. The lower the value of the ratio, the highest denitrification can be achieved. Acknowledgement--The authors wish to thank Professor Y. Argaman of the Technion Israel Institute of Technology, Haifa (Israel) for his kind suggestions during the discussion of the results and the data elaboration. REFERENCES

Abufayed A. A. and Sehroeder E. D. (1986) Kinetics and Stoichiometry for SBR/denitrification with a primary sludge carbon source. J. Wat. Pollut. Control Fed. 58, 398-405. APHA, AWWA and WPCF (1989) Standards Methods for the Examination of Water and Wastewater, 17th edition. American Public Health Association, Washington, D.C. Argaman Y. and Brenner A. (1986) Single-sludge nitrogen removal: modeling and experimental results. J. Wat. Pollut. Control Fed. 58, 853-860.

Bitcover E. H., Cooper J. S. and Bailey D. G. (1980) Pilot plant study on suspended solids flooculation of lime-sulphide unhairing effluents. J. Am. Leather Chem. Ass. 75, 108-118. Ekama G. A. and Marais G. v. R. (1984) Theory Design and Operation of Nutrient Removal Activated Sludge Process. Wat. Res. Commission, Pretoria. Ghimenti G. and Botrini C. (1988) La depurazione degli effiuenti di conceria: un bilancio dei resultati ottenuti e le prospettive che si inquandrano nel rispetto dei limiti di legge. In Acque Reflue e Fanghi (Edited by Frigerio A.), Vol. 1, pp. 308-322. Milano, Italy. Keenan J. D., Steiner R. L. and Fungaroli A. A. (1979) Substrate inhibition of nitrification. J. envir. Sci. Hlth, Part A envir. Sci. Engng A14, 377-397. McClintock S. A., Sherrard J. H., Novak J. T. and Randall C. W. (1988) Nitrate versus oxygen respiration in the activated sludge process. J. nam. Pollut. Control Fed. 60, 342-350. Panzer C. C. (1982) Biological nitrogen control--A comparison of methods. J. Am. Leather Chem. Ass. 77, 149-160. Panzer C. C., Komanowsky M. and Senske G. E. (1981) Improved performance in combined nitrification-denitrification of tannery waste. J. Wat. Pollut. Control Fed. 53, 1434-1439. Pastore R., Nobili E., Miriani E. and Vallero P. (1984) Some experiences in tannery waste waters treatment. Cuoio Pelli Mater. Concianti 60, 286-291. Rozich A. F. and Castens D. J. (1986) Inhibition kinetics of nitrification in continuous-flow reactors. J. Wat. Pollut. Control Fed. 58, 220-226. Sutton P. M., Bridle T. R., Bedford W. K. and Arnold J. (1981) Nitrification and denitrification of an industrial wastewater. J. Wat. Pollut. Control Fed. 53, 176-184. Szpyrkowicz L., Rigoni-Stern S. and Zilio Grandi F. (1991) Pilot plant studies on tannery wastewater treatment with the objective to reduce sludge production. Wat. Sci; Technol. 2,3, Water Pollution Research and Control-Kyoto 1990, Part 4, pp. 1863-1873. Vismara R. (1982) Depurazione Biologica, Teoria e Processi (Edited by Hoepli U.), p. 321. Milano, Italy.