Groundwater recharge of sewage effluent through amended sand

War. Res. Vol. 26. No. 3, pp. 285-293. 1992 Printed in Great Britain. All rights tx'served

00,13-1354/92$5.00+0.00 Copyright~ 1992PergamonPress pk

GROUNDWATER RECHARGE OF SEWAGE EFFLUENT THROUGH AMENDED SAND GOEN E. Ho tO, RoavN A. GIBBSIO, KURUVILLAMATt~W j@ and WILLIAMF. P~atra~-2 Environmental Science, Murdoch University, Murdoch 6150 and 2Parker and Associates, P.O. Box 175, North Perth 6006, Western Australia (First receit,ed January 1991; accepted in revised form August 1991) Abstract--The performance of a groundwater recharge basin at the Kwinana Groundwater Recharge Site in Western Australia was monitored between 1983 and 1986. A primary aim of the monitoring programme was to study the improvement in the removal of faecal coliforrus and nutrients (nitrogen and phosphorus) by amending the sand of the recharge basin with gypsum-neutralized red mud (fine bauxite refining residue). The study consisted of five operating stages. Stage 1 was a baseline study using unamended sand. Stages 2-5 were after sand amendment with ted mud. Continuous flooding and flooding/drying regimes were studied with primary effluent or a mixture of primary and secondary effluents. Phosphorus removal was maintained at a high level (over 80%) in all of the stages after the sand amendment. Faecal coliform removal was generally excellent, except at the beginning of each stage when primary effluent was used, and only a thousand-fold reduction was achieved. Removal improved with time and most monitored bore samples contained no faecal coliforms/100ml. With one exception the groundwater met water quality criteria for irrigation. Nitrogen removal of approx. 45% was obtained with primary effluent using a cycle of flooding and drying (stage 3). Continuous flooding with primary effluent (stage 5) did not improve denitrification. No nitrogen removal was observed with a mixture of two-thirds secondary effluent and one-third primary effluent. Key wora~--groundwater recharge, red mud, sewage effluent, faecal coliforms, coliphages, total coliforms,

nitrogen, phosphorus

INTRODUCTION Groundwater recharge systems using wastewater are useful both as a treatment device and to replenish groundwater reserves. However, they are also a potential source of groundwater pollution. Artificial recharge systems need to be designed to provide maximum removal of wastewater pollutants such as microbial pathogens and nutrients (nitrogen and phosphorus). An important aspect of groundwater recharge system design is the soil infiltration basins. Not all soils are adequate for pollutant removal. The suitability of the sands of Perth, Western Australia for groundwater recharge have been examined previously. The removal of ammonia-nitrogen by passage through Bassendean sand, the major sand type in the Perth area, was found to be negligible (Ho et aL, 1981; Mathew et al., 1982). A mathematical model was developed to simulate nitrogen removal from wastewater by passage through soil, and Spearwood sand (another Perth sand) was found to be suitable, if amended with a loam soil. The use of Spearwood and Bassendean sands for sewage and waste disposal was the subject of a number of other investigations (Ho et al., 1981; w,t a/~-c

Whelan et al., 1981; Parker and Mce, 1982). The results indicated that Bassendean sand was inadequate for the efficient removal of pollutants such as enteric bacteria, viruses and nutrients (or potential pollutants such as heavy metals). By comparison, Spearwood sand, with a small percentage of clay, was relatively more efficient at removing enteric bacteria. The evidence suggests that both these soils can be significantly improved by providing more reactive materials, as well as an increased surface area. The model developed by Ho et al. (1981) suggests that the addition of clay material to the sands of the Swan Coastal Plain should improve the removal of wastewater pollutants. The object of this study was to test this theory in a field trial by exploiting the fines fraction of the residue from bauxite refining (red mud). A substantial proportion of this residue is the iron oxide material goethite. The resultant soil mixture was expected to conform to a sandy loam and thus to have much higher removal efficiencies. The study was conducted at the Kwinana Waste Water Treatment Plant experimental recharge basin in Western Australia. Laboratory based column experiments were also carried out and the results have been reported previously (Kayaalp et al., 1988; Ho et al., 1989, 1991).

285

Goe~ E. Ho et al.

286

Table I. Properties of Spearwood sand and red mud

Speeurwood Red Parameter Effective size (ram) Uniformity coe~lcient Particle density (&~cm3) Saturated hydraulic conductivity (m/day) Cation exchange capacity (NH, saturation method, mequiv/100g)

sand 0.18 |.9 1.55 16 1.5

mud 0.002 4.8 1.1 0.0003 41

MATERIALS AND METHODS

Sand and red mud Spearwood sand and red mud were used in this study. Properties of these are shown in Table I. Red mud was obtained from the Kwinana refinery of Alcoa of Australia. Western Australia. The high pH and alkalinity of the residue meant that it needed to be neutralized before use for soil amendment (Ho. 1989). Several neutralizing agents were tested, and waste gypsum from a nearby superphosphate fertilizer plant was found to be ideal for the purpose, as the calcium also displaces sodium from the exchange sites of the red mud. Five percent by weight of the gypsum decreased the pH of the red mud to 8.3 equivalent to the pH of calcareous soils (Ho et al., 1985; Wong and Ho, 1988), Groundwater recharge site The layout of the basins and observation bores at the Kwinana Groundwater Recharge Site is shown in Fig. I. Basin number I was employed because it was equipped with subsurface sampling pans. Table 2 shows the depths of the pans and bores. The regional groundwater flow at the recharge site was generally from the east to the west. When the basin was flooded a groundwater mound was created beneath the basin, and it was expected that there was a flow away from the mound in all directions superimposed on the regional groundwater flow. B2 was upstream of the recharge basin. As a control observation well it always showed very low concentrations

ofall the parameters measured. B3, BI, B7, 136 and B8 were downstream of the basin with a gradient away from the basin roughly in the indicated order. All these bores were constructed to penetrate the groundwater to a depth of I m below the estimated lowest water table position, with the casing slotted over the I m depth. Bores 3, 6, 7 and 8 were added after stage 1 of the programme was completed. Wastewater for recharge was obtained from an adjacent wastewater treatment plant serving a predominantly residential area with a population of approx. 8000. The effluent was treated using primary sedimentation and the secondary activated sludge process. The primary effluent had a typical biochemical oxygen demand (BOD) of 200 mg/I and suspended solids concentration of 130mg/l. The secondary effluent had a typical BOD of 20 mg/l and suspended solids concentration of 10 mg/l.

Description of study Stage 1: baseline study. February and March 1983. (Basin unamended Spearwood sand. Hooded with one-third primary and two-thirds secondary effluent.) Samples of bore water and liquid from five subsurface pans were collected during 4 flooding (9 days) and drying (7 days) cycles over 2 months. For the first 2 cycles, samples from pans were collected by allowing liquid to collect in the pans and pipe work and sampled via taps. In the last 2 cycles the pans were allowed to drain freely, thus avoiding possible bacterial die-off, and overnight flow collected. All samples were transported on ice and assayed within 3-4h for microbial activity or frozen before being analysed for chemical content. Samples were also collected from two bores, one upstream (B2) and one downstream (BI) of the basin. Samples were assayed for total coliforms, faecal coliforms, coliphage, nitrogen and phosphorus. Repacking of basin with sand amended with red mud. After an initial delay due to the unavailability of an appropriate gypsum-neutralized red mud, the basin was packed with 30% by weight red mud in August 1984. One metre of sand was removed from the base of basin number i. A mixture of sand and red mud was prepared using a soil mixer, and a I m layer was placed in the basin in increments of about Fence

/

Groundv,oter / t

J

10metres but ion

Effluent ,~ /

hEk~sN ni oR I MC~

] L_[~_JBosin No2 ~

"~'.Samp/ing pans and manhole

Ui, Rc~dwQy

Fig. I. Kwinana groundwater recharge site.

Groundwater recharge through amended sand Table 2. Depths of pans and bores Depth (m) Pans PI 0.75 P2 1.25 P3 2.25 P4 3.0 P5 3.75 Bores BI 15.1 B2 14.5 B3 ! 5,4 B6 16.5 B7 16.9 B8 17.2

0.2 m using a bob-cat/grader. The performance of the basin was monitored from September 1984 to January 1985. The operation of the basin was stopped because the infiltration rate was low (less than 0.1 m/day) due to lower than expected hydraulic conductivity compared to laboratory tests. It was likely that this was caused by uneven layered packing, with some layers experiencing over compaction. The total red mud-sand mixture was removed from the basin and remixed with more sand to give a red mud content of 20%. The lower percentage of red mud was designed to prevent any layer having more than 30% red mud from non-uniform mixing, thus avoiding clogging. The mixture was dropped into the basin using a conveyer belt to prevent machinery compacting the mixture in the basin. Stage 2. March-June 1985. (Continuous flooding with one-third primary and two-thirds secondary effluent.') Flooding was recommenced in March 1985 with one-third primary and two-thirds secondary effluent. Flooding was carried out continuously with the aim of promoting anaerobic conditions beneath the basin and increasing denitrification. The rate of infiltration was initially approx. I m/day but decreased with time. At the end of 106 days of flooding, the infiltration rate was approx. 0.1 m/day. From May 1985 bore samples were obtained by both pumping and bailing, to ensure that samples were not taken from stagnant water. Total coliform analysis was discontinued due to time constraints. As only nitrification was observed, even with continuous flooding, it was decided to use only primary effluent. This was to enhance both the organic carbon supply necessary for the beterotrophic denitrifiers and to promote anaerobic conditions by providing a substrate with a higher oxygen demand (BAD). A leak was, however, detected in the valve supplying secondary effluent to the basin, so that primary effluent by itself could not be supplied to the basin. The basin was allowed to dry with secondary effluent continuously leaking to the basin. Samples from boreholes and pans continued to be taken during this period. After the leak was repaired stage 3 of basin operation was commenced. Stage 3. August 1985-March 1986. (Flooding and drying with primary effluent.) Primary effluent was applied to the amended basin on a cycle of 9 days flooding and 12 days drying. A longer drying period than in stage ! was chosen to allow for comparable drying in the amended soil. A flooding--drying regime, rather than continuous flooding, was used as an intermediate step between flooding and drying with secondary effluent and continuous flooding with primary effluent, to see if denitrification could be achieved. Nine flooding cycles were completed over a 222 day period. Coliphage monitoring was discontinued due to time constraints. Breakthrough of bacteria was noticed in two observation bores downstream of the basin, and it was decided to test whether this was due to the use of primary effluent or to previous accumulation of bacteria in the soil (stage 4). Stage 4. March 1986-July 1986. (Flooding and drying with one-third primary and two-thirds secondary ef~uent.)

287

A mixture of one third primary and two thirds secondary effluent was used with a 9 day flooding and 12 day drying period (4 flooding cycles over 133 days). Stage 5. July 1986-September 1986.(Continuous flooding with primary effluent.) Primary effluent was continuously flooded over the 56 day period to see if denitrification could be enhanced by both promoting anaerobic conditions beneath the basin and supplying organic carbon to the denitrifying bacteria.

Application rate of effluent During flooding, effluent was applied daily and the application rate was estimated by noting the drop in the level of effluent in the basin. More effluent was then added to make up to the original level (100 cm), except near the end of the flooding period when the basin level was allowed to drop until the drying period commenced. There was usually a decline in the application rate over a flooding period. The range of application rates is indicated in Table 3 for the 5 stages of basin operation.

Sampling Samples were taken weekly from the basin, from pans located beneath the basin, and from 6 bores around the basin. Bore samples were taken by bailing, and from May 1985 were also obtained by pumping, taking a sample after flushing two volumes of water originally in the bore. For obtaining representative samples, sampling by pumping was desirable. However, for bacterial analysis it was considered that pumping might cause contamination between the bore samples. Samples were analysed for ammonium-N, nitrate-N, phosphate-P and faecal coliforms. Samples from the bores taken by pumping were only analysed for nutrients,

Microbiological analysis Total coliforms and faecal coliforms were enumerated by the membrane filtration method using m-endo and mFC media, respectively (APHA, 1975). Coliphages were assayed using a mixed-indicator host. A two stage process was used employing the most probable number technique (Kott, 1966) and plaque assay (Parker, 1981).

Analytical procedures Phosphorus concentration as phosphate-phosphorus was determined by the Murphy and Riley method (1962). Col. orimetric determination was performed on a Shimadzu Table 3. Application rates of effluent Measured

application rate Stage I 2 3

4

5

Flooding period Feb.-Mar. 1983 (4 flooding and drying cycles) 12 Mar.-19 June 1985 (continuous flooding)

6-15 Aug. 1985 27 Aug.-5 Sept. 1985 23 Aug.-I Sept. 1985 Longer drying period to rejuvenate basin 23---30 Oct. 1985 13-22 Nov. 1985 2-1 i Dec. 1985 13-22 Jan. 1986 3-12 Feb. 1986 25 Feb.-5 Mar. 1986 17-25Mar, 1986 7-16 Apr. 1986 28 Apr.-5 May 1986 23 June-2 July 1986 28 July 1986-19 Sept. 1986 (continuous flooding)

(cm/day) ~200 100--*10 100-,25

55--.10

45--*5 70--*30 65--*20 80--+30 60--.15 60.*20 60-+15 60--.15 60-,-15 60-*20 40--.10 35--.10

GOF~ E. Ho

288

UV-210A double-beam spectrophotometer at a wavelength of 882 nm and slit width of 15 nm using a 4 cm cell. Samples were prepared as described by Kayaalp et al. (1988). Ammonium as nitrogen and nitrate as nitrogen were analysed using a Technicon AutoAnalyzer II using standard methods (APHA, 1975). RESULTS Stage !. Baseline study

et

al.

Nutrient results are presented in Table 4. There was a greater removal as the depth of soil increased but it did not follow a regular pattern. The rates of flow in the sample pans were not uniform and this might be the cause of irregularities in the results. Only one sample was collected daily, but the influent concentration might have varied during the day, so it was possible to detect a higher pan concentration than basin concentration on a particular day. Nitrogen and phosphorus concentrations in the well upstream of the flow of groundwater (B2) were low. In the downstream well (BI) 35% of nitrogen and 75% of phosphorus concentration reduction occurred compared to the effluent. About 30-45% of nitrogen and 30-40% phosphorus reduction occurred between the pans and the wells. There was a general trend for the reduction of nitrogen and phosphorus. As the primary and secondary effluents were mixed together, the presence of organic carbon resulted in nitrate undergoing denitrification. There was algal growth in the basin which may also have removed some nitrogen and phosphorus. Even though the reduction in nitrogen was only 30-35% it was significant, as nitrate is difficult to remove. The reduction in phosphorus was approx. 35% and the remaining concentration was still about 6-7 rag/I, which is still a high value. Removal of phosphorus in such a system is very difficult unless soil with a very high adsorption for phosphorus is added.

(Unamended sand. Flooding and drying with onethird primary and two-thirds secondary effluent.) Table 4 shows the geometric means of positive samples, for faecal coliforms, total coliforms and coliphages, in samples taken from effluent, subsurface pans and the downstream bore BI. No indicator organisms were found in the upstream bore (B2). Total coliform and faecal coliform numbers were reduced substantially during infiltration but complete removal was not achieved. The results were very variable. During two flooding cycles the faecal coliform counts for BI ranged from 50 to 4.75 x 105/100 ml, but for two other cycles generally no faecal coliforms were detected. The patterns for total coliforms were the same. Coliphage reduction was not as efficient although the virus concentrations were generally tower than coliform concentrations. BI was sampled on 18 occasions during the 4 cycles and was positive on 12 of these. Positive samples ranged between 2 and 2.5 x 10~ plaque forming units/100 ml. No observations of daily fluctuations in flow rate for sampling pans were made, but pan 2 had a higher flow rate, presumably accounting for higher counts. In general, removal of bacteria and viruses (col- Stage 2 iphage) during infiltration through this soil was not (Red mud gypsum-amended sand. Continuous efficient. The total extent of travel could not be flooding with one-third primary and two-thirds sec. reliably determined from the data presented, but it is ondary effluent.) reasonable to assume that sewage organisms might be Table 5 shows a summary of the results of the detected beyond bore BI. High counts for sampling nutrient analyses for the period March-June 1985 pans may be accounted for in part by high flow rates, and faecal coliform and coliphage determinations for especially for pan 2. The results obtained in this study the same period. were not entirely consistent with earlier studies using After repacking the red mud in the basin, all the same soil (Parker, 1983). In the study of septic sampling pans beneath the basin flowed freely. The tanks a considerable barrier ("crust") was always removal of both faecal coliforms and coliphage was observed, leading very likely to unsaturated flow not complete, but was better by several orders of conditions at shallow depths and thus greater re- magnitude than for sand alone (stage 1). Pans 1, 4 moval occurred. In this present study there appears and 5 gave higher readings than pans 2 and 3 and the to have been a lack of hydraulic barrier at the differences were likely due to different infltration effluent-soil interface. rates above the pans (channelling). Table 4. Results of nutrient (arithmetic mean) and miarobiolosical (geometric mean) analyses (stage I)

NH,-N (mg/l) Basin Pan I Pan 2 Pan 3 Pan 4 Pan $ Bore I Bore 2

21.6 13.6 15.7 7.9 5.7 1.5 4.4 0.6

N D - none detected.

NO)-N PO,-P Total coliforms (mg/1) (rag/l) (count/100ml) 6.7 4.2 1.2 4.6 5.3 11.2 10.9 0.5

9.3 6.7 8.9 5.1 4.7 5.2 2.3 0.1

3.9 x 106 3.7 x 103 1.9 x l0 s 1.9 x 104 1.9 x 10J i.2 x 104 290 ND

Faecal coliforms (count/100ml) 4.4 x 105 2.3 x 104 1.7 x 10~ 290 390 92 220 ND

Coliphages (count/100ml) 900 97 84 71 44 34 46 ND

Groundwater recharge through amended sand

289

Table 5. Results of nutrient (arithmetic mean) and microbiololl~d (geometric nwan) analyses

(stage 2) NH4-N NO3-N PO.-P (m&~t) (mR/l) (re&l) Basin Pan I Pan 2 Pan 3 Pan 4 Pan 5 Bore I Bore 2 Bore 3 Bore 6 Bore 7 Bore 8

17.1 18.5 10.5 0.9 12.1 25.9 0.2 0.3 0.2 0.2 0,3 0.2

3.8 4.9 13.5 20.3 9.3 5.0 23.4 0.2 7.6 13.4 6.0 10.6

5.5 2.0 2.7 2.5 3.5 3.3 3.8 0.2 2.4 2.9 0.9 0.1

Faecal coliforms (count/t00ml) 1.2 x I0S(13/13)" 1.7 x 104(12/13) 8.3 (4/13) 300 (3/13) 26(2/13) 110(12/13) 1.4 (2113) ND (0/13) ND (0/13) 1.3(5/15) ND (0/13) ND (0/13)

Cofipbages (count/100ml) 1.9 x 10~(9/9) 160(7/10) 4.6 (4110) 5.4 (3/10) 6.7(5/10) 22(8/10) 4.2 (3/10) 2(I/10) 35(I/10) ND (0/10) ND (0/10) ND (0/10)

"In parentheses: number of positive samples/total number of samples. ND = none detected.

Almost no faecal coliforms or coliphages were detected in bore samples. Out of 13 samples from each bore, only on 5 occasions were faecal coliforms detected in either bore I or bore 6. These were either I or 2 faecal coliforms/100 ml. Only on 4 occasions were coliphages detected in bores 1 or 3. These values ranged from 2 to 35 coliphage/100ml, which is again at a low level and considerably less than for unamended sand. Ammonium-N was detected in pans i and 5 in concentrations as high as in the basin, which indicates that channelling to these pans took place and is consistent with the bacterial counts noted above. Samples from pans 2 and 3 had lower ammonium-N, but the sum of ammonium-N and nitrate-N was the same as for the effluent in the basin, thus indicating that nitrification had taken place but there was no denitrification. Continuous flooding of the basin still resulted in nitrification but there was no denitrification from the expected anaerobic conditions under continuous flooding. Primary etfiuent contains organic-N which breaks clown to ammonium-N in the recharge basin or during soil percolation. On the other hand some N would be taken up by algae and bacteria. Since organic-N was not measured in the study the addition of N from the breakdown of organic-N has not been included. The conclusions drawn from the results of the study would not however be invalidated, since any estimated removal of N by denitrification would be a conservative estimate. There was very little ammonium in all the bores sampled. A likely explanation is nitrification of ammonium during effluent travel, and is confirmed by high nitrate concentrations in bore 1. Bores 3 and 7, which were adjacent to bore 1, had lower nitrate concentrations after the bores were pumped for sampling. The reduction in phosphate concentration ranged from 30% for BI to 98% for B8. The average reduction for all pans and bores was 59%. There was a considerable decrease in phosphate concentration with passage in the groundwater but only 64% reduction occurred beneath the basin.

Stage 3 (Flooding and drying with primary effluent.) The primary effluent contained a high concentration of ammonium, very low nitrate, and relatively high phosphate and faecal coliforms (Table 6). The organic carbon content of primary effluent was much higher than secondary effluent and it was expected that denitrification would be improved. This expectation was borne out by the results of the bore analyses. The ammonium-N concentration downstream of the basin was low (less than I rag/I), and the average nitrate-N concentration of B3 and B I was 25 rag/I, while of B6 and B8 it was 24 mg/I. Nitrogen removal of about 45% had taken place with a minimum of 18% for the worst situation (Bl). The primary effluent would have contained some organicN, and therefore the removal rate would have been higher than estimated above. The ammonium-N content of samples Pl to P5 was relatively high (average 18 rag/I), and suggests that the adsorption capacity of the red mud for ammonium was exceeded and that nitrification was not complete in the red mud and sand layers. The level of nitrates in PI and P5 was very high (average 43 rag/I), such that the sum of ammonium-N and nitrate-N exceeded the sum of ammonium-N and nitrate-N for the effluent in the basin. The high average figures were due to the occurrence of nitrateTable 6. Results of nutrient (arithmetic mean) and microbiological (geometric mean) analyses (stage 3)

NH,-N NO3-N PO4-P (mR/l) (mR/l) (mR/l) Basin Pan I Pan 2 Pan 3 Pan 4 Pan 5 Bore I Bore 2 Bore 3 Bore 6 Bore 7 Bore 8

45 20 7.6 2.2 28 34 0.5 0.4 0.2 0.7 0.3 0.1

0.6 88 59 47 15 5.8 37 0.4 13 30 1.8 18

9.4 0.8 1.6 1.4 2.2 1.4 2.6 0.1 1.9 2.4 0.2 0.1

Faecal coliforms (count/100ml) I.I x IOS(ll/ll) * 3.6 x 104(11/12) 12.2(7/15) 2.9 (3/20) 5.2(10120) 57(14/20) 10.4(14/20) 0.5 (I/20) 160(2/20) 1.5 (7/20) 1.5 (1/20) 0.6(3/20)

"In parentheses: number of positive samples/total number of samples.

290

G o ~ E. Ho et aL

peaks, very high concentrations of nitrate-N, following the start of flooding of the basin. The peaks reached over 300 mg/I nitrate-N and biased the average concentration upwards. Differences existed between the behaviour of liquid flow to the sampling pans, resulting in a gradient from Pl to P5 contrary to an expected regular increase or decrease in concentration. The differences were observed previously, and persisted through the monitoring period. The most probable cause was different flow paths and infiltration rates from the basin to the sampling pans. It is likely that PI, P4 and P5 had relatively rapid liquid flowing into them and P2 and P3 relatively slow flowing liquid. By the time the liquid reached the bores the unevenness had been averaged. Further nitrification had taken place resulting in very low ammonium-N in all bores. Phosphorus removal was high (average 84%) and higher than observed in the previous stage. The reason for this was likely a reduction in phosphate leaching in the sand below the basin by the initial alkalinity released by the red mud. The removal of phosphorus must have taken place in the red mud as there was little further reduction in phosphorus concentration from the sampling pans to the bores downstream of the basin. The use of primary effluent resulted in a high degree of faecal coliform breakthrough in the pans (PI, P2 and P5) and also to a lesser extent in bores BI and B3 at the beginning of stage 3. The high readings were associated with pan samples obtained during the flooding period. The removal of faecal coliforms generally improved with time and in the last 3 months very few faecal coliforms were detected except in PI. It was nevertheless decided to change from the use of primary effluent to mainly secondary effluent to determine if faecal coliform counts in the bores could be reduced. Stage 4

(Flooding and drying with one-third primary and two-thirds secondary effluent.) The mixture of two-thirds secondary effluent and one-third primary effluent still had a relatively high ammonium content and a low nitrate concentration (Table 7). The phosphate and faecal coliform concentrations were comparable to the primary effluent used in stage 3, although on average faecal coliform concentrations were smaller in stage 3. There was a marked reduction in faecal coliform concentration when the effluent reached the sampling pans and bores. Only Pl had significant faecal coliform counts and the high counts were associated with samples taken during flooding periods. All other sampling pans had very low or zero counts. The relatively high average count in BI was due to one single high count from a sample taken after high rainfall during a drying period of the basin. The above results show that the use of secondary effluent

Table 7. Results of nutrient (arithmetic mean) and microbiological (geometric mean) analyses (stage 4)

NH,-N NO3-N PO.-P Basin Pan 1

(m~l)

(mg~l)

38

I.!

4.6 64 Pan 2 1.2 37 Pan 3 0.2 62 Pan 4 0.5 31 Pan 5 8.3 33 Bore I 3.2 33 Bore 2 0.5 0.6 Bore 3 0.5 33 Bore 6 14 25 Bore 7 0.5 0.2 Bore 8 I.I 15 *In parentheses: number of samples. ND - none detected.

(mg/I) 11

Faecal coliforms (count/t00 ml) 3.0 x 10~(9/9) *

0.3 1.0 x 10J(12/15) 1.3 3.3 (3/14) 1.6 i.4 (2/18) !.1 0.5(I/18) 0.6 0.5(2/18) 4.0 2(4/18) 0.1 ND (0/18) 0.4 I (7/18) I.I 1.3(2/18) 0.4 0.6 (5/18) 0.2 1.8(9/18) positive samples/total number of

decreased the faecal coliform counts in the sampling pans and bores. It seems likely, however, that over a long period of operation there would be little difference in the faecal coliform counts whether primary or secondary effluent was used, because the data indicate that faecal coliform removal improved with time. The formation of an organic mat in the basin could explain the improvement in bacterial removal. The ammonium concentration in the sampling pans was lower than in stage 3, and the decrease was more than the reduction in ammonium concentration in the basin. A possible explanation for this is the lower build up of organic slime in the soil in the basin with the use of secondary effluent, allowing more air to move into the soil pore promoting nitrification. Nitrate peaks were observed in samples collected from the sampling pans, as was the case during stage 3. The average ammonium concentrations in BI and B6 were high which is difficult to explain. Earlier results (Tables 5 and 6) for the ammonium concentration were low and an explanation is the ease of nitrification of the ammonium (even under continuous flooding of the basin, during March-June 1985). Based on the results of nitrogen analyses for B6 no nitrogen removal took place, whereas based on the average of BI and B3 only 8% nitrogen removal occurred. The lack ofdenitrification is consistent with the use of secondary effluent, and lower supply of organic carbon. When compared to the use of two-thirds secondary effluent and one-third primary effluent, but with continuous flooding of the basin (March-June 1985), there was less ammonium in PI-P5. This can be explained by nitrification taking place during the drying period with flooding and drying. There was, however, more nitrate in PI and P5. An explanation is from the enhanced nitrification and the peaks resulting from intermittent flooding. No apparent denitrification was observed when employing continuous flooding using mainly secondary effluent. The removal of phosphate-P in red mud continued to be excellent.

Groundwater recharge through amended sand

Stage 5 (Continuous flooding with primary effluent.) Continuous flooding with primary effluent was expected to enhance nitrogen removal, as denitrificalion would be increased by both a high supply of organic carbon and the promotion of anaerobic conditions when the soil was continuously wetted. The results shown in Table 8 indicate that the degree of nitrogen removal was similar to stage 3 and not higher. A detailed analysis of the results suggests that in and below the I m red mud amended soil the flow was unsaturated, enabling oxygen diffusion from the surrounding area and preventing anaerobic conditions developing over the entire flow path of the effluent in the soil. PI showed a high ammonium concentration, but the other sampling pans showed very low ammonium concentrations. These concentrations were lower than those obtained in stage 3 with flooding and drying of the basin using primary effluent. The lower infiltration rate when continuous flooding was used could explain the higher degree of ammonium oxidation beneath the l m red mud amended sand. No nitrate peak was observed, because of continuous flooding of the basin. Evidence of denitrification is indicated by the results of nitrogen analysis of samples from the sampling pans (ammonium-N plus nitrate-N in the pans was less than ammonium-N plus nitrate-N in the basin). No further denitrification appeared to take place between the sampling pans and bores. The concentration of ammonium-N in BI and B6 was still declining at the end of the monitoring period and a steady state situation might not have been reached. The phosphorus removal indicated by the sampling pans was still excellent (average over 85%). Samples from B I had higher phosphate concentrations than in the sampling pans in this and the previous stage (Table 7). This is difficult to explain, unless there was leaching of phosphorus from the soil between the pans and BI, which is rather unlikely. A more likely explanation is channelling to BI. Table 8. Results of nutrient (arithmetic mean) and microbiological (geometric mean) analyses (stage $) NH4-N NOj-N PO4-P Faecal coliforms (mE/I) (mg/I) (ms/I) (count/100 ml) Basin Pan I Pan 2

42 17 0.5

0.5 12 10

10 0.4 1.3

2.9 x 106(7/7) " 1.6 × 10J(5/6) I (I/7)

Pan 3 Pan 4

Pan 5 Bore I Bore 2 Bore 3

0.4 0.4 0.2 2.4 0.4 0.5

14 26 33 23 0.3 29

1.5 1.3 1.3 `1.5

ND (017) ND (0/7) 0.~f(0]7) ,17(2/7)

0.1

ND (0/7)

Bore 6 Bore 7 Bore 8

12 I,I 0.2

23 0.7 17

1.5 0.1 0.6

2.5(I/7) ND (0/7) ND (0/7)

0.4

9.3 (2/7)

*In parentheses: number of positive ,rumples/total number of samples. ND - none detected,

291

The removal of faecal coliforms was almost as good as in stage 4, and better than the average removal in stage 3, when primary effluent was used in a flooding drying regime. PI had a high faecal coliform content (Table 8), but there was a thousand-fold reduction between the basin and PI. Very few faecal coliforms reached P2-PS. B! and B3 had relatively high average faecal coliform counts due to one high faecal coliform count in each case amongst other nil counts. In the case of B 1 it occurred at the beginning of the stage, and did not recur. With B3 it occurred during week 5 of the stage, and appeared to be an isolated breakthrough of faecal coliforms. It is likely based on these and other observations discussed in previous sections that isolated breakthroughs could be expected, but that in general, even with the use of primary effluent, soil filtration would remove most of the faecal coliforms during long regular operation of a recharge basin. DISCUSSION AND CONCLUSIONS The general aim of the project was to test the hypothesis that amending the sand at the Kwinana Recharge Site with red mud would improve the removal of bacteria and nutrients from the effluent. Bacterial removal should be improved by filtration, die-off and adsorption, phosphorus removal by adsorption and nitrogen removal by nitrificationdenitrification.

Faecal coliform removal Faecal coliform removal in the amended sand was generally much better than removal in unamended sand. Only at the initial period of stage 3 was there significant breakthrough, but this decreased considerably as the flooding-drying cycles progressed. The field monitoring results paralleled the results of a laboratory study (Ho et al., ! 991), which showed that bacteria and viruses were better removed in red mud amended sands than unamended sands, This was primarily caused by increased die-off due to lower infiltration rates, but filtration (straining) and adsorption were also operative.

Phosphorus removal Phosphorus removal was excellent (generally greater than 80%) over the period of the monitoring programme, which was expected based on laboratory batch and column tests (Kayaalp et al., 1988). Phosphate-P is removed by adsorption. Although the adsorption capacity of the red mud would eventually be exhausted, this should not occur at the Kwinana Recharge Site for a number of years.

Nitrogen removal The best nitrogen removal rate (of approx. 45%) obtained in the current programme was during stage 3, when primary effluent was used with a flooding and

292

G o ~ E. Ho et al.

drying regime. Continuous flooding using primary gratefully acknowledged. The encouragement of Mr Don Glennister (Manager, Residue Development, Alcoa) and Mr effluent did not improve denitrification (stage 5). Hugh Rule (Senior Engineer, Water Authority of Western Previous work has shown that in nitrogen removal Australia) is greatly appreciated. by bacterial nitrification and denitriflcation, adsorption of ammonium onto the red mud during a REFERENCES flooding period is followed by nitrification of adsorbed ammonium to nitrate. The nitrate is denitrifled in a subsequent flooding period, accompanied A P H A 0975) Standard Methods for the Examination of Water and Wastewater, 14th edition. American Public by removal of ammonium from the effluent by adHealth Association, Wash/ngIon D.C. sorption to the red mud (Mathew et al., 1982). Bouwer H. (1991) Ground water recharge with sewage effluent. ;Vat.Sci. Technol. 23, 2099-2108. It appears that at the recharge site, the above processes took place in the 1 m of sand amended by Carlson R. R., Linstedt K. D., Bennett E. R. and Hartman R. B. (1982) Rapid infiltration treatment of primary and red mud. In the sand below the red mud unsaturated secondary effluents. J. Wat. Pollut. Control Fed. 54, flow occurred, allowing air diffusion which resulted in 2?0-280. further nitrification. No further denitrification took Hart B. T. (1974) A Compilation of Australian Water Quality Criteria. Australian Water Resources Council, Technical place because of the oxidative condition and lack of Paper No. 7. Australian Government Publishing Service, organic carbon. This hypothesis appears to explain Canberra. the nitrogen removal results obtained. Ho G. E. (1989) Overcoming the salinity and sodicity of red Other explanations are, however, possible, as the mud for rehabilitation and reuse. In 43rd Purdue Indus. trial Waste Conference Proceedings, pp. 641--649. Lewis, baseline study in 1983 (before the basin was amended Chelsea, Mich. by red mud) did not include the use of primary Ho G. E., Gibbs R. A. and Mathew K. (199[) Bacteria and effluent. virus removal from secondary effluent in sand and red A higher degree of nitrogen removal appears to be mud columns. War. Sci. Technol. 23, 26[-270. achievable by optimizing the lengths of the flooding Ho G. E., Mathew K. and Newman P. W. G. ([98[) Groundwater recharge using treated sewage. Suitability and drying periods. It is also possible that greater of soils of the Swan Coastal Plain, Western Australia for dcnitrification could have been achieved by lower nitrogen removal. In Proc. of Groundwater Recharge infiltration rates. Bouwer (1991) reported that total Conference, Australian Water Resources Council, Confernitrogen removal increased from 30 to 7-0% when ence Series No. 3, pp. 215-232. Australian Government Publishing Service, Canberra. infiltration rates were decreased from 120 to 70 m/year. In this study infiltration rates ranged from Ho G. E., Mathew K. and Newman P. W. G. (1989) Leachate quality from gypsum neutralized red 18 to 365 m/year but were generally between 55 and mud applied to sandy soils. War. Air Soil Pollut. 47, 220 m/year. 1-18. The use of primary effluent gave greater nitrogen Ho G. E., Newman P. W. G., Mathew K. and de Potter H. (1985) Neutralization of bauxite processing residue with removal than a mixture of primary and secondary copperas. In Proc. 13th Australian Chemical Engineering effluent. There was initially some breakthrough of Conference, pp. 103-108. Institution of Engineers, faecal coliforms into the groundwater with the use of Australia. primary effluent, but after 2 weeks operation counts Kayaaip M., Ho G., Mathew K. and Newman P. (1988) Phosphorus movement through sands modified by red never exceeded 200/ml. This removal is better than mud. Water 15, 26-29, 45. reported in some previous studies which found conKott Y. (1966) Estimation of low numbers of Escherichia centrations of 1000/ml (Bouwer, 1991) and an avercoil bacteriophage by use of the most probable number age of 6.3 × 10S/ml (Carlson et al., 1982), in shallow method. Appl. Microbiol. 14, 141-144. wells. In a comparison of four rapid infiltration sites, Leach L. E., Enfield C. G. and Harlin C. C. Jr (1980) Summary of Long-Term Rapid Infiltration System Studies. Leach el al. (1980) reported mean faecal coliform EPA-600/2-80-I65, U.S. EPA, Ada, Okla. concentrations in groundwater ranging from less than Mathew K., Newman P. W. G. and Ho G. E. (1982) 1-1.86× 105/ml. However, comparisons between Groundwater Recharge With Secondary Sewage E~uent, these studies are difficult because of differences in soil Australian Water Resources Council, Technical Paper No. 71. Australian Government Publishing Service, types, infiltration rates, depths of bores and groundCanberra. water levels. J. and Riley J. P. (1962) A modified single solution The results of this study suggest that primary Murphy method for the determination of phosphorus in natural effluent is preferable to the use of secondary effluent waters. Analyt. chim. Acta 27, 31-36. for artificial recharge. Previous studies have reached Parker W. F. (1981) A note on the use of membrane faecal coliform medium for enhancing resolution and accuracy the same conclusion (Leach et aL, 1980; Carlson when enumerating a small plaquing coliphage. J. appl. et al., 1982; Rice and Bouwer, 1984). Bact. Sl, 81-84. The groundwater produced by this recharge system Parker W. F. (1983) Microbial aspects of septic tank effluent would be suitable for irrigation but not for drinking disposal into coarse sands in the Perth Metropolitan water [based on Australian standards summarized by Area. Department of Conservation and the Environment, Western Australia (Bulletin 130). Hart (1974)]. Parker W. F. and Mee B. J. (1982) Survival of Salmonella adelaide and fecal coliforms in coarse sands of the Swan Acknowledgements--The financial support of Alcoa of AusCoastal Plain, Western Australia. Appl. envir. Microbiol. tralia and the Water Authority of Western Australia is 43, 981-986.

Groundwater recharge through amended sand Rice R. C. and Bouwer H. (1984) Soil-aquifer treatment using primary effluent. J. Wat. Pollut. Control Fed. 56, 84-88. Whelan B. R., Barrow N. J. and Carbon B. A. (198[) Movement of phosphate and nitrogen from septic tank effluent in sandy soils near Perth, Western Australia. In Proc. Australian Water Resources Council Conference "Groundwater Pollution", 1979 (Edited by Lawrence

293

C. R. and Hughes R. J.), pp. 391-401. Australian Government Publishing Service, Canberra. Wong G. W. C. and Ho G. E. (1988) Neutralization and cation dissolution characteristics of bauxite refining residue. In Hazardous and Industrial Wastes (Edited by Varma M. M. and Johnson J. H. Jr), pp. 247-264. Hazardous Materials Control Research Institute, Silver Spring, Md.