Random-packed anaerobic filter in piggery wastewater treatment

Biological Wastes 20 (1987) 157-166

Random-Packed Anaerobic Filter in Piggery Wastewater Treatment W. J. N g & K. K. C h i n Department of Civil Engineering, National University of Singapore, Kent Ridge, Singapore 0511 (Received 25 April 1986; accepted 14 August 1986)

A B S TRA C T The anaerobic filter has been used on a laboratory scale to treat piggery wastewater. Screened wastewater was digested in a 21-litres effective volume reactor random-packed with plastic media. Feed wastewater was introduced into the filter in an upflow mode at regular intervals using a timer-controlled pump at loading rates representing hydraulic retention times ( H R T ) o f 6"3 to 2"1 days. COD reductions ranged from 97% to 83% of influent COD while VSS reductions ranged from 99% to 90%. Transient-state gas quality in terms of methane content was very stable. Methane content in the biogas increased as retention times decreased and ranged from 75% to 84% methane. Results indicate that a high potential reduction of required digester volume is possible through application of thefixed-film concept.

INTRODUCTION Increases in the cost of energy in recent years have stimulated the search for wastewater treatment processes which are more energy efficient. To this end, anaerobic processes have attracted considerable interest. Anaerobic digestion processes are energy efficient because they do not need to transfer large quantities of oxygen into the wastewater. Sludge management requirements are also reduced because the process produces substantially less biological solids than conventional aerobic treatment processes. In addition, the methane-rich biogas generated by the process is a convenient energy source for plant operation. 157 Biological Wastes 0269-7483/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

158

w.J. Ng, K. K. Chin

Wastewaters arising from confined livestock operations, because of their high organic strength, are well suited to anaerobic digestion. Treatment of the wastewater commonly begins with separation of the solids fraction from the liquid. The solids fraction is then anaerobically digested while the liquid fraction undergoes aerobic treatment. Anaerobic processes are usually limited by the low growth rate of the methanogens. Due to this limitation, conventional suspended-growth anaerobic treatment systems require lengthy retention times and thus large reactor volume. It would therefore be economically less attractive if the liquid fraction, which is usually lower in COD, were also treated anaerobically. With the introduction of digester configurations other than the conventional suspended-growth single-reactor system, the means for reducing reactor volumes without sacrificing treatment efficiencies may be available. These digester configurations allow retention of microorganisms, including the methanogenic bacteria, in the digester allowing it to be operated at significantly shorter hydraulic retention times. Some variations of these configurations include the sludge-bed upflow digester and the anaerobic filter (Mosey, 1978; Lettinga et al., 1979). In the anaerobic filter, bacteria adhere to support media so that, even at relatively high hydraulic loads which would wash bacterial biomass out of conventional suspended growth digesters, the filter retains the bacteria (Young & McCarty, 1967). As a result of this, the anaerobic filter's removal efficiencies show little sensitivity to daily fluctuations in influent wastewater quality (Kobayashi et al., 1983). In the agro-industrial sector, the anaerobic filter has been successfully used to treat screened dairy manure and distillery wastewaster (Lo et al., 1984; Russo et al., 1985). This study reports work done with a random-packed anaerobic filter used to treat piggery wastewater. The specific objectives of the study were: to examine the stability of the process at short hydraulic retention times; to examine its treatment efficiencies and to compare process parameters and performances with other anaerobic processes for the treatment of piggery wastewater.

METHODS Feed material

The piggery wastewater was collected from a commercial pig farm where the pig wastes were flushed from the pens and channeled into a ditch. The ditch was grossly overloaded and, because of solids accumulation, its hydraulic retention time was not easily determined. The ditch contents were obviously

Anaerobic filter for pig wastewater

159

undergoing anaerobic digestion. Effluent from this ditch was collected and screened with a 2 m m sieve to remove gross particles like grit and bristles before feeding into the filter. Feed material was collected twice a week and material not immediately used was stored at 4°C. The average Chemical Oxygen Demand (COD) of the feed used to test the filter at various hydraulic retention times ranged from 7688 to 13 789 mg litre-1. Five-day Biochemical Oxygen Demand (BODs) ranged from 568 to 713mglitre -1. The Volatile Suspended Solids (VSS) of the feed was more consistent at 6100 to 6847 mg litre- 1. Alkalinity and pH ranged from 1998 to 4834 mg litre- 1 and 7.55 to 7.84, respectively.

Anaerobic filter The cylindrical, packed-bed filter was fabricated from plexiglass, with an overall height of 140cm and an internal diameter of 19 cm. The support media consisted of lengths of PVC tubing 25 m m long, 12 m m diameter and 1 m m wall thickness, and the filter was filled to a depth of 105 cm with the plastic media above a perforated liquid-dispersion plate. This resulted in a void volume of 30 litres. The filter was seeded with screened sludge from an anaerobic digester of a domestic wastewater treatment plant. When the biofilm was in place, the effective volume of the filter was 21 litres. Piggery wastewater was introduced into the filter through its bottom for 15 min in every 45 min using a timer-controlled peristaltic pump. Mixing within the filter was provided by recirculating the entire contents of the filter four times an hour by withdrawing the filter liquor from the top and returning it to the bottom. Effluent was withdrawn from the top of the filter by a pump. Systematic analysis of performance data began when the biofilm was firmly established after 360 days. Gas produced was collected by displacing acidified water and daily production measurements were made after equilibrating the gas to atmospheric pressure.

Analytical methods Feed and filter effluent samples were collected at regular intervals and VSS, COD, BOD 5 and total Kjedahl nitrogen (TKN) measured according to procedures in Standard M e t h o d s (APHA, 1980). COD, BOD 5 and T K N were measured without separation of the suspended solids fraction from the liquid. Influent and effluent pH were measured with a D K K meter. Gas quality was measured in terms of percentage methane, carbon dioxide and nitrogen. Samples of the gas were chromatographed on a 2 m long Porapak Q 80/100 mesh column and the elutants analyzed with a thermal conductivity detector.

160

W. J. Ng, K. K. Chin

RESULTS A N D DISCUSSION Substrate removal

The filter demonstrated excellent COD removal at all the hydraulic retention times (HRT) tested (Table 1). Although there was a general trend of decreasing COD removal efficiency as the hydraulic loading rate increased, this was not distinct. The COD removal efficiency of the filter remained relatively stable at about 95% and showed a significant drop to 83% only when the HRT was changed to 2.1 days. Such performance surpassed that of the two-phase system investigated by the authors in an earlier study (Ng et al., 1986). System performance was then between 74% and 78% COD removal at 11 days HRT. The greater efficiency of the filter was contributed to by the lower concentration of VSS in the effluent. The BOD s removal efficiency of the filter was as good as its COD removal, at between 87% and 93%. Effluent BOD 5 was, in general, lower than 100 mg TABLE !

Influent and Effluent Characteristics and Filter Performance Characteristics

Hydraulic retention time (days) 6"3

Influent, TSS (mg litre- 1) Effluent, TSS (mg litre-1) TSS % reduction Influent VSS (mg litre-1) Effluent VSS (mg litre- 1) VSS % reduction Influent COD (mg litre-1) Effluent COD (mg litre-1) COD % reduction Influent BOD (mg litre-1) Effluent BOD (mg litre-1) BOD % reduction Influent COD/BOD ratio Effluent COD/BOD ratio Influent T K N (mg litre-1) Effluent T K N (mg litre- 1) T K N % reduction Influent alkalinity (mg litre-1) Effluent alkanity (mg litre-1) Influent pH Effluent pH

13 150 69 99"5 6607 34 99"5 10372 461 96 736 94 87 14:1 5:1 925 316 66 1 896 1 728 7.67 7.28

4"9

14 120 52 99"6 6 370 33 99-5 9 137 254 97 568 40 93 16:1 6:1 614 177 71 2 022 1 032 7'66 7'61

3"5

2.8

14690 13 290 30 223 99"8 98'3 6847 6 750 18 160 99"7 97.6 10873 13 789 613 729 94 95 713 706 66 52 91 93 15:1 19:1 9:1 14:1 929 1232 412 462 56 63 3 776 4 834 2 198 2 623 7.84 7'75 7-39 7'78

2.1

10090 1187 88"2 6 100 594 90.0 10368 1 678 84 918 118 87 11:1 14:1 925 509 45 3 312 2 391 7.78 7-73

Anaerobic filter for pig wastewater

161

litre- 1 when the HRT was greater than 2.8 days. With some polishing step using aerobic treatment the effluent would be expected to reach the standard of 50 mg litre-1 BOD5 for discharge into watercourses in Singapore and Malaysia. Suspended solids in the feed wastewater were effectively removed by the filter. The mechanism for suspended solids removal must, in the first instance, have involved a filtering action whereby the solids were retained within the filter media. Since the filter did not suffer from clogging problems, some of the retained solids must have undergone biodegradation and gasification. Total Suspended Solids removal was better than 98 %, although this dropped to 88% during the period of the shortest HRT. The retained material was being washed out of the filter at this HRT but the solids were well mineralized and settled readily. The feed wastewater was dark brown in colour, but upon settling the resulting effluent was clear with a greyish tint. Upgrading this water for reuse within the farm with the sequencing batch reactor (SBR) is a distinct possibility (Chin & Ng, 1985).

Gas production The retained Volatile Suspended Solids also served as a substrate reserve resulting in an effluent quality which was relatively stable in comparison with influent quality, which fluctuated over a wide range. The quality of the gas produced was also stable over the duration of the investigation (Fig. 1). This is in agreement with the findings of Lo et al. (1984) and Kennedy & van den Berg (1982). The methane content of the gas was high, at between 75% and 84% (Table 2), the highest values being obtained at the lowest HRT. This high methane content was contributed to by the dissolution of carbon dioxide into the liquor of the filter. Gas yields did not vary substantially for HRT of 4-9 to 2-8

TABLE 2 Gas Composition and Yield

Hydraulic retention time (days)

Gas yield (litres per gram COD removed)

CH 4 (%)

CO 2

N2

(%)

(%)

6-3 4-9 3'5 2'8 2-1

0-17 0-25 0"20 0'24 0-53

75 75 77 84 84

7 7 10 12 15

16 16 13 4 1

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30

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120

130

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Anaerobic filter for pig wastewater

163

days. However, there was a doubling in gas yield when the H R T was reduced to 2.1 days. Gas yields were found to be substantially higher than the values reported by Oleszkiewicz (1983) for an anaerobic upflow biofilter and Yang & Chou (1985) for a horizontal baffled anaerobic reactor. The filter, however, could not tolerate shock washout of its biofilm. This was indicated at the time when the H R T was changed from 3.5 to 2.8 days. The recirculation rate was accidentally increased to 30 change-overs of bed volume per hour, resulting in a large increase in suspended solids in the effluent over a few hours. Although the fault was corrected, COD removal efficiency did not fully recover until 6 days later while gas quality only regained stability after 24 days (Fig. 1). Also of interest was the nitrogen content of the gas. This declined rapidly when the H R T was reduced from 3"5 to 2-8 days and eventually to 2.1 days. While this may be indicative of a shift in species dominance in bacteria population, a more likely reason would be a gas leak in the system, especially as the nitrogen content was rather high at 13%-16%. Although the average feed quality showed little variation from one phase of the investigation to the next, transient quality showed large variations. An example of this is shown in Fig. 2. The stability of the filter in the face of such large organic load variations was good, with effluent COD and VSS fluctuating only between 800-2100 mg litre- 1 and 200-1000 mg litre- 1. It is noteworthy that effluent COD remained relatively stable during periods when feed COD concentrations dropped. This would indicate the filter, by retaining materials, was capable of damping out feed-quality variations in either direction.

Operational characteristics and design criteria While it was initially feared that the packed-bed might trap the gas produced, this was found not to be the case in practice as the recirculation flow successfully dislodged the bubbles of gas. The recirculation flow also served to completely mix the liquor within the filter. Thus samples drawn from various heights of the filter all indicated essentially similar characteristics. This was markedly different from the behaviour of filters described by other workers (Young & McCarty, 1967; Sachs et al., 1982; Russo et al., 1985). An examination of the distribution of retained material in the filter at various heights from its base indicated that most of the accumulation occurred in the middle portion with the lightest accumulation at the top. This was unlike filters without the recirculation flow where major accumulation of material can be expected in the bottom portion of the filter bed. Although no provision was made for desludging the filter, it would be

W . J . Ng, K. K. Chin

164 16000

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prudent to incorporate this facility in any full-scale plant. Table 3 shows a mass balance of the Fixed Suspended Solids (FSS). The removal of FSS was exceptionally high. This material could have been removed from the influent by retention in the filter, as a dissolved material in the effluent and as a suspended material in the effluent. At the end of the study the amount of FSS TABLE 3 Mass Balance Based on Fixed Suspended Solids Concentrations

End o f each H R T phase

6-3 4"9 3"5 2"8 2'1

Cumulative

Cumulative

Cumulative FSS

influent

effluent

removed or

removed or

FSS (g)

FSS (g)

retained (g)

retained (%)

777 1 964 3 324 4 888 5 813

98-9 99-1 99"3 98'4 92-5

785 1 981 3 346 4 965 6 282

8 17 22 77 469

Cumulative FSS

165

Anaerobic filter for pig wastewater TABLE 4

Comparison of Performance Characteristics of Some Examples of Fixed-Film Reactors Parameters and performance

Type of reactor Operational temperature (°C) HRT (days) SRT (days) COD loading (g litre- 1 day- 1) Methane yield (litre g- 1 COD added) COD removal efficiency(%)

Yang & Chou (1985)

Oleszkiewicz (1983)

This study

Horizontal baffled 30 + 1 2.0 120

Anaerobic upflow biofilter 23 1.0 50

Anaerobic filter with recirculation 30 _+1 2.1 --

2'1

4.0

5'0

0-45 58

0"10 73

0.44 84

removed or accumulated in the filter and lost as dissolved substances was assessed at 5813 g. Unfortunately, the two values could not be separated as Total Solids was not a parameter routinely measured. Increasing the recirculation rate momentarily was observed to be effective in dislodging the attached biofilm. This could then be removed with the effluent. Separating this material from the liquor should not present any difficulties as it was observed to settle well. Assuming that the results of this study could be applied at a farm level, the potential for reducing digester volume would be significant. This is especially so when the filter is compared to the mesophilic suspended growth digesters. However, the filter described here is also comparable in performance with fixed-film reactors reported elsewhere (Table 4). It is noteworthy that such performance was achieved at C O D loading rates which were significantly higher than those reported by Yang & C h o u (1985) and Oleszkiewcz (1983).

REFERENCES APHA (1980). Standard methods for the examination o f waste and wastewater, 15th edn. APHA, AWWA, WPCF, Washington, DC. Chin, K. K. & Ng, W. J. (1985). Bioxidation and denitrification study using the sequencing batch reactors. In: Advances in water engineering (Tebbutt, T. H. Y. (Ed.)), Elsevier Applied Science Publishers, 248-53. Kennedy, K. J. & van den Berg, L. (1982). Anaerobic digestion of piggery waste using a stationary fixed film reactor. Agricultural Wastes, 4, 151-8.

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W. J. Ng, K. K. Chin

Kobayashi, H. A., Stenstorm, M. K. & Mah, R. A. (1983). Treatment of low strength domestic wastewater using the anerobic filter. Water Res., 17(8), 903-9. Lettinga, G., van Velsen, L., de Zeeuw, W. & Hobma, S. W. (1979). The application of anaerobic digestion to industrial pollution treatment. Proc. First International Symposium on Anerobic Digestion, Cardiff University, Applied Science Publishers, 167-87. Lo, K. V., Whitehead, A. J., Liao, P. H. & Bulley, N. 1~. (1984). Methane production from screened dairy manure using a fixed-film reactor. Agricultural Wastes, 9, 175-88. Mosey, F. E. (1978). Anaerobic filtration: A biological treatment process for warm industrial effluents. J. Wat. Pollut. Control, 3, 370-6. Ng, W. J., Chin, K. K. & Wong, K. K. (1986). Anaerobic fermentation in agroindustrial wastes treatment. Int. Conf. on Water and Wastewater Management in Asia, Singapore, Feb. 26-28. Oleszkiewicz, J. A. (1983). A comparison of anaerobic treatments of low concentration piggery wastewaters. Agricultural Wastes, 8, 215-31. Russo, C., Sant'Anna Jr., G. L. & de Carvalho Pereira, S. E. (1985). An anaerobic filter applied to the treatment of distillery wastewaters. Agricultural Wastes, 14, 301-13. Sachs, E. F., Jennett, J. C. & Rand, M. C. (1982). Pharmaceutical waste treatment by anaerobic filterl J.A.S.C.E., 108, EE2, 297-313. Yang, P. Y. & Chou, C. Y. (1985). Horizontal-baffled anaerobic reactor for treating diluted swine wastewater. Agricultural Wastes, 14, 221-39. Young, J. C. & McCarty, P. C. (1967). The anaerobic filter for waste treatment. Proc. 22nd Ind. Waste Conf., Purdue University, 559-74.