- Email: [email protected]
Evaluation on a pilot-scale attached-growth pond system treating domestic wastewater
~
Wat. Res. Vol. 30, No. 1, pp. 242-245, 1996
Pergamon
0043-1354(95)00134-4
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/96 $9.50 + 0.00
RESEARCH NOTE E V A L U A T I O N ON A PILOT-SCALE A T T A C H E D - G R O W T H P O N D SYSTEM T R E A T I N G DOMESTIC W A S T E W A T E R Q I N G L I A N G Z H A O * and B A O Z H E N W A N G @ Water Pollution Control Research Center, Harbin University of Architecture and Engineering, 144 Dazhi Street, Harbin 150006, P.R. China
(First received July 1994; accepted in revised form April 1995) Abstract--Pilot-scale experiments on attached-growth ponds (AGP) had been conducted to investigate the feasibility of improving waste stabilization ponds (WSP) performance through the incorporation of attached-growth media (AGM) or the so-called artificial fibrous carriers in the pond water. Better removal efficiencies of COD, BOD 5 and NH 4 N had been obtained in the AGP than in the conventional WSP through the comparative studies. On the basis of these results, a pilot-scale AGP system had been built and run, which had shown to have great advantages and stabilities with better effluent quality in performance in comparison with conventional WSP.
Key words--attached-growth ponds (AGP), microbe; performance evaluation, waste stabilization ponds
bacteria and algae. D. S. Sparker (1988) regarded the attached-growth treatment systems show considerable promise for space and cost reductions, many of the newer reactor configurations are in the development stage and need to be extended to full scale and evaluated. A G P is just a new-developed technology, about which there are only a few studies (Shin and Polprasert, 1987; Shin and Polprasert, 1988; Polprasert and Charnpratheep, 1989) and applications in practice (Qi et al., 1991), showing the better effluent quality could be expected.
INTRODUCTION
The development of wastewater treatment technologies is on the one hand toward high-technological fields, and on the other substantially toward low-cost, energy-saving, easy-operation and maintenance aspects or the so-called innovative and alternative technologies (Ellis, 1990; Wang 1990; Wang 1991). As in the latter case, waste stabilization pond (WSP) has great reputation for its low capital investment, less energy consumption and operation costs, and therefore has found great application in municipal and industrial wastewater treatment. According to statistics, there had already existed over 7000 WSP in the United States (U.S. Protection Agency, 1983), more than 3500 in Europe distributing in 16 countries (Vuillot and Boutin, 1987), and above 10,000 available both in urban and in rural areas in China (Wang, 1990) in use and its number is still increasing. However, because of the main drawbacks of WSP such as longer hydraulic detention time and low microorganism density, large land area requirement for pond construction is needed, which limit its further utilization. Moreover, there have been great needs for the improvement of pond performance for organic pollutant removal. This study was undertaken to investigate the feasibility of improving WSP performance and reduction of land areas through the addition of attached-growth media ( A G M ) or the so-called artificial fibrous carriers in the pond water, through which A G M serves as habitat for the attached-growth of
EXPERIMENTAL DESIGN AND M E T H O D S
The experiments were conducted using three pilot-scale ponds with working dimensions of 4.0 m in length, 1.2 m in width and 1.1 m in depth, which were constructed and located in the New Campus of Harbin University of Architecture and Engineering under ambient conditions. The attached-growth media (AGM) adopted in this study are patentable SB-Type products or artificial fibrous carriers consisting of fine strings of polyvinyl acetate, with specific surface area of 1236 m2/m3 and cost only RMB 50 Yuan/m 3, which has recently been developed and produced by Yuhuan Environmental Protection Equipment Factory in Zhejiang Province, China. The raw domestic wastewater from the residential areas was taken out by a submersible pump (Q = 5 m3/h, H = 5 m) from the inspection shaft of sewers near experiment sites and was fed continuously to the ponds or pond system. The operation of three pilot-scale AGP was in series to form a pond system, of which AGM was added 22% of the effective pond volume in the first two ponds and no AGM in the third one. The flow rate was controlled at 2.0 m3/d thus the total retention time of such a pond system was 7.5 days. The schematic profiles of AGP system in series including specific details were shown in Fig. 1.
*Author to whom all correspondence should be addressed. 242
Research Note
Biochemical analyses and microbiological determination Bio-filmthickness on the artificial carriers was determined microscopically by placing a fibre under microscope equipped with a micrometer. Temperature, light intensity, dissolved oxygen (DO) and pH value were measured by Thermometer, Illumination meter, DO meter (0-20 mgO2/1) and pH meter (0-14.00) respectively. Other analyses as COD, BODs, NH4-N and microbiological examination were undertaken according to the procedures outlined in Standard Methods (APHA, 1980), as well as in Analytical Methods (NEPA, 1989). EXPERIMENTAL
RESULTS
AND
DISCUSSION
Determination of optimum A G M density (fl) The operation of three pilot-scale AGP was firstly run in parallel for comparative study for organic removals in determination of optimum AGM density. The density of AGM (fl) for the three ponds was varied at 0, 11 and 22% of the effective pond volume (V = 5.0m3), and the flow rates of influent were controlled at 0.7 m3/d thus theoretical retention time for each pond was about 7 days. Better removal of organic pollutants had been obtained in the pond packed with AGM, occupying 22% of the pond volume during the comparative studies. This could be illustrated by COD, BOD5 and NH4-N reduction which was shown as follows. When AGM density
was varied at 0, 11 and 22% of the effective pond volume, COD was removed 10.5-63.5%, 25.0-66.2% and 46.1-70.1%, BOD5 was removed 62.1-85.3%, 67.2-90.1% and 72.2-95.7%, and NH4-N was removed averagely 24.8, 29.5 and 39.7% respectively.
Bio-film, biomass and biospecies Because of the higher attachment and growth rate of biomass, thickness of bio-film growth on the artificial fibrous carriers was found to linearly increase with time during the initial stage of operation (up to 20 days) in pond 1 and pond 2. The bio-film thickness later became constant, being about 30-40 ~m on the lower parts of the carriers under the water and 70-100 ~m on the upper parts of carriers near the surface of pond water. Owing to the possible SS attachment to the AGM, a ~small increase in bio-film thickness had been observed near the inlet of pond system. After some periods of time, the film reached such a thickness that the substrate was utilized before penetrating the entire depth of the microbial film. The microorganisms that existed in the starvation zone would utilize their own cytoplasmic material to maintain life-support functions, i.e. they were in the endogenous growth phase. In
4 9
/~ ~l 5
6 ! ! ! ! I
! ! ! ! !
243
t
-/
(a) Plain layout
.~.
(b) Vertical section Fig. 1. AGP system in series. 1. AOP l(fl~ = 22%). 2. AGP 2(fl2= 22%). 3. Pond 3(fl3= 0). 4. Wastewater tank (2600 x 900 x 500). 5. Submersible pump. 6. Inspection shaft. 7. Influent pipes (D = 25 mm). 8. Effluent pipes (D = 25 ram). 9. Control valves. 10. Artificial SB-type fibrous carriers. 11. Connection pipes (D = 25 ram).
244
Research Note
addition, there existed external shear forces around bio-film caused by wind, rainfall and water flow velocity variation. Therefore, the microorganisms in this phase lost the ability to cling to the fibrous support materials and detached themselves from the supports and settle down to the pond bottom. Because there are great difficulties to describe microbe quantatively by using bio-film thickness, biomass is usually adopted for this purpose. Biomass can be obtained by putting some other bunches of AGM into ponds and taking a bunch of AGM out of pond in a definite period of time, by washing them and weighing the detached mass both in wet and dry conditions. It could be known from these tests that biomass increased progressively up to 15-30 days, then became constant. The volatiles within the biofilm formed on AGM in steady state are 80.0-86.5% in pond 1 and 83.1-91.3% in pond 2 respectively, which will represent the quantities of biomass. As is known to all, bacteria are the main decomposer of organic pollutants in the conventional suspended-growth ponds and play a vital role in the natural stabilization processes, being widely utilized for the treatment of organic wastewaters. They are also the dominant species in AGP system, which may also illustrate the bio-film activities on some degree. The experimental results of bacteria examination are 5 x 105 MPN/gAGM(wet) in pond 1 and 9 x 104MPN/gAGM (wet) in pond 2 respectively, and with the vertical distribution of larger amounts on AGM near the water surface. In addition to large quantities of bacteria attached on the AGM, a vast species of algae and protozoa as well as metazoa or some higher animals are also attached and grown on the artificial fibrous carriers to form various food chains. It can be known from the microscope-examined results of algae and protozoa that algal counts are higher in pond 2 than those in pond 1, except for Euglena. Of the 15 algal species, Oscillatoria, Euglena, Chlorella and Nitzschia are the dominant species in both pond 1 and pond 2. They and any other algae presented in the AGM utilize the end-products such as CO2 and NH4-N of bacterial metabolism from attached- or suspendedgrowth phases, synthetize algal cells and release oxygen to the pond water. Thus every slice of fibrous carriers, especially those near the water surface, serve as small aerators in the day-time. This is the reason that so many oxygen bubbles appear on the surface of AGM. With regard to protozoa and metazoa there are quite abundant of microbe such as Vorticella and Epistylis in the attached fibrous carriers, which may not appear in the conventional suspended-growth ponds. Of greatest interests is that there are also large amount of higher animals such as Rotifers, Roundworms and Daphnia which also play an important role in purification of wastewater, because Rotifers can effectively consume dispersed or coagulated bacteria and small granual organic matters to make effluent clarified, and they usually serve as indicators showing
quite effective aerobic treatment of wastewater having been realized. Roundworms are the main predators of bacteria and protozoa rather than soluable organic materials, which appear only in the water environment of enough dissolved oxygen. Bio-film formed in the attached-growth media is the ideal food sources to Roundworms, by which bio-film become loose, detaches from support media and renews continuously so that oxygen transfer increases and film has vigourous activity. Daphnia is a minute crustacean which predates mainly small particles such as bacteria, single-cell algae, protozoa and the decayed organic materials. When there are enough food sources and DO, it proliferates and grows quite rapidly in the temperature of 15-25°C. From the statements above, both Roundworms and Daphnia are the indicators to show that very low organic pollutants and stable water quality, high DO level and effective treatment of domestic wastewater by such a AGP system.
Benthal deposit As in the conventional WSP, the sludge accumulated in the AGP system is also involved in the overall treatment process due to the detachment of biofilm from AGM and to the anaerobic digestion of sedimented organic matters as well as trapping of undegradable solids. In order to know some information about the benthal deposit in AGP system, five culture vessels (D = 105 mm) were equidistantly put into pond 1 and pond 2 near the bottoms on July 20 respectively, hang-taken out and analysed, which gave the results that sludge accumulation rate ranged from 4.62 to 6.58 mm in pond 1 and from 3.10 to 4.48 mm in pond 2 monthly. The values of deposit depth and other parameters for pond 1 were some greater than those for pond 2, which was simply because of carryover of inorganic solids from influent of domestic wastewater. Volative percentages in the deposit for pond 2 are greater than those for pond 1, which may imply that organic matters or detached bio-film from AGM or dead microorganisms as algae deposited in the subsequent pond.
pH Values and DO levels As routine analyses, pH and dissolved oxygen are monitored almost every day. While hourly monitoring results of typical pH and DO content, conducted on August 5 for the AGP system, had shown that variation of DO is typical of that usually expected in the cyclical increase and decrease during day and night periods for all the three ponds, respectively. DO levels increase from pond 1 to pond 3 progressively, which may be contributed to the organic pollution becomes lighter and lighter, and maximum DO concentrations in pond 3 reaches about two times the sturation level (above 20 mg/l). With regard to pH variation, the rise in DO level is accompanied consistently by a simultaneous increase in pH. In general, increased algal activities are accompanied by in-
Research Note creased pH during day hours, which reaches 7.85 in pond 1, reaches 8.40 in pond 2 and 9.30 in pond 3, respectively.
245
authors wish to express their thanks to Ms Shumei Chen for her efforts to the analytical works and technical assistance during the study. Acknowledgement--The
COD, B O D s and N H 4 - N reduction f r o m A G M system
The purpose of incorporating AGM into ponds is the maximum reduction of organic pollutants in terms of COD, BOD5 and NH4-N reduction. It has been found from the experimental results that COD and BOD5 removal in such a AGP system are quite satisfactory. According to the statistical data from analyses, COD and BOD 5 removals in the AGP system are 75.6% (range from 50% in winter to 91.4% in summer) and 90.2% (range from 49% in winter to 95.9% in summer) respectively. With regard to NH4-N, its removal can be expected from 30.3% to 95.9%, dependent also upon temperature. These correspond with the experimental results from a laboratory studies (Zhao, 1991).
CONCLUSION The incorporation of attached-growth media (AGM), or artificial fibrous carriers used here in the study, enhanced the performance of conventional waste stabilization ponds by the formation of a great number of small stable ecological systems around AGM, being abundant of biospecies from bacteria and algae to protozoa and higher animals, increasing the biomass, improving the biological distribution, which have been confirmed by the study of pilot-scale AGM system. The experimental results also illustrate that satisfactory effluent quality can be expected in such a AGP system, although the total detention time of the system has been controlled and shortened to only 7.5 days during operation.
REFERENCES
APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th edn. Am. Pub. Health Assoc., Washington, D.C. Ellis K. V. (1990) Trends in wastewater treatment. Developing Worm Water. Grosvenor Press International. NEPA (1989) Anatytical Methods for Water and Wastewater, 3rd edn. Chinese Environmental Publishing House, Beijing (in Chinese). Parker D. S. (1988) Wastewater technology innovation for the year 2000. J. environ. Engng, Am. Soc. Cir. Engrs 114, 487 505. Polprasert C. and Charnpratheep K. (1989) Heavy metal removal in attached-growth waste stabilization ponds. Wat. Res. 23, 625-631. Qi P. S., Wang B. Z., Ma F., Zhang J.S. and Li T. J. (1993) Intensification of a pond by fibrous carriers. Wat. Sci. Technol. 28(7), 117-123. Shin H. K. and Polprasert C. (1987) Attached-growth waste stablization pond treatment evaluation. Wat. Sci. Technol. 19(12), 229-235. Shin H. K. and Polprasert C. (1988) Ammonia nitrogen removal in attached-growth Ponds. J. Environ. Engng, Am. Soc. Cir. Engrs 114, 846 863. U.S. Environmental Protection Agency (1983) Design Manual--Municipal
Wastewater
Stabilization
Ponds.
EPA625/I-83-105, Cincinnati, Ohio. Vuillot M. and Boutin C. (1987) Waste stabilization ponds in Europe~a state of art review. Wat. Sci. Technol. 19 (12), 1-6. Wang B.Z. (1990) Water Pollution Control Engineering. High Education Press, Beijing (in Chinese). Wang B. Z. (1991) Ecological waste treatment and utilization system--a low-cost, energy-saving/generating and resources recoverable technology for water pollution control in China. Wat. Sci. Technol. 24(5), 8-19. Zhao Q. L. and Zhang Z. J. (1991) Temperature influence on performance of oxidation ponds. Wat. Sci. Technol. 24(5), 85-96.
Wat. Res. Vol. 30, No. 1, pp. 242-245, 1996
Pergamon
0043-1354(95)00134-4
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/96 $9.50 + 0.00
RESEARCH NOTE E V A L U A T I O N ON A PILOT-SCALE A T T A C H E D - G R O W T H P O N D SYSTEM T R E A T I N G DOMESTIC W A S T E W A T E R Q I N G L I A N G Z H A O * and B A O Z H E N W A N G @ Water Pollution Control Research Center, Harbin University of Architecture and Engineering, 144 Dazhi Street, Harbin 150006, P.R. China
(First received July 1994; accepted in revised form April 1995) Abstract--Pilot-scale experiments on attached-growth ponds (AGP) had been conducted to investigate the feasibility of improving waste stabilization ponds (WSP) performance through the incorporation of attached-growth media (AGM) or the so-called artificial fibrous carriers in the pond water. Better removal efficiencies of COD, BOD 5 and NH 4 N had been obtained in the AGP than in the conventional WSP through the comparative studies. On the basis of these results, a pilot-scale AGP system had been built and run, which had shown to have great advantages and stabilities with better effluent quality in performance in comparison with conventional WSP.
Key words--attached-growth ponds (AGP), microbe; performance evaluation, waste stabilization ponds
bacteria and algae. D. S. Sparker (1988) regarded the attached-growth treatment systems show considerable promise for space and cost reductions, many of the newer reactor configurations are in the development stage and need to be extended to full scale and evaluated. A G P is just a new-developed technology, about which there are only a few studies (Shin and Polprasert, 1987; Shin and Polprasert, 1988; Polprasert and Charnpratheep, 1989) and applications in practice (Qi et al., 1991), showing the better effluent quality could be expected.
INTRODUCTION
The development of wastewater treatment technologies is on the one hand toward high-technological fields, and on the other substantially toward low-cost, energy-saving, easy-operation and maintenance aspects or the so-called innovative and alternative technologies (Ellis, 1990; Wang 1990; Wang 1991). As in the latter case, waste stabilization pond (WSP) has great reputation for its low capital investment, less energy consumption and operation costs, and therefore has found great application in municipal and industrial wastewater treatment. According to statistics, there had already existed over 7000 WSP in the United States (U.S. Protection Agency, 1983), more than 3500 in Europe distributing in 16 countries (Vuillot and Boutin, 1987), and above 10,000 available both in urban and in rural areas in China (Wang, 1990) in use and its number is still increasing. However, because of the main drawbacks of WSP such as longer hydraulic detention time and low microorganism density, large land area requirement for pond construction is needed, which limit its further utilization. Moreover, there have been great needs for the improvement of pond performance for organic pollutant removal. This study was undertaken to investigate the feasibility of improving WSP performance and reduction of land areas through the addition of attached-growth media ( A G M ) or the so-called artificial fibrous carriers in the pond water, through which A G M serves as habitat for the attached-growth of
EXPERIMENTAL DESIGN AND M E T H O D S
The experiments were conducted using three pilot-scale ponds with working dimensions of 4.0 m in length, 1.2 m in width and 1.1 m in depth, which were constructed and located in the New Campus of Harbin University of Architecture and Engineering under ambient conditions. The attached-growth media (AGM) adopted in this study are patentable SB-Type products or artificial fibrous carriers consisting of fine strings of polyvinyl acetate, with specific surface area of 1236 m2/m3 and cost only RMB 50 Yuan/m 3, which has recently been developed and produced by Yuhuan Environmental Protection Equipment Factory in Zhejiang Province, China. The raw domestic wastewater from the residential areas was taken out by a submersible pump (Q = 5 m3/h, H = 5 m) from the inspection shaft of sewers near experiment sites and was fed continuously to the ponds or pond system. The operation of three pilot-scale AGP was in series to form a pond system, of which AGM was added 22% of the effective pond volume in the first two ponds and no AGM in the third one. The flow rate was controlled at 2.0 m3/d thus the total retention time of such a pond system was 7.5 days. The schematic profiles of AGP system in series including specific details were shown in Fig. 1.
*Author to whom all correspondence should be addressed. 242
Research Note
Biochemical analyses and microbiological determination Bio-filmthickness on the artificial carriers was determined microscopically by placing a fibre under microscope equipped with a micrometer. Temperature, light intensity, dissolved oxygen (DO) and pH value were measured by Thermometer, Illumination meter, DO meter (0-20 mgO2/1) and pH meter (0-14.00) respectively. Other analyses as COD, BODs, NH4-N and microbiological examination were undertaken according to the procedures outlined in Standard Methods (APHA, 1980), as well as in Analytical Methods (NEPA, 1989). EXPERIMENTAL
RESULTS
AND
DISCUSSION
Determination of optimum A G M density (fl) The operation of three pilot-scale AGP was firstly run in parallel for comparative study for organic removals in determination of optimum AGM density. The density of AGM (fl) for the three ponds was varied at 0, 11 and 22% of the effective pond volume (V = 5.0m3), and the flow rates of influent were controlled at 0.7 m3/d thus theoretical retention time for each pond was about 7 days. Better removal of organic pollutants had been obtained in the pond packed with AGM, occupying 22% of the pond volume during the comparative studies. This could be illustrated by COD, BOD5 and NH4-N reduction which was shown as follows. When AGM density
was varied at 0, 11 and 22% of the effective pond volume, COD was removed 10.5-63.5%, 25.0-66.2% and 46.1-70.1%, BOD5 was removed 62.1-85.3%, 67.2-90.1% and 72.2-95.7%, and NH4-N was removed averagely 24.8, 29.5 and 39.7% respectively.
Bio-film, biomass and biospecies Because of the higher attachment and growth rate of biomass, thickness of bio-film growth on the artificial fibrous carriers was found to linearly increase with time during the initial stage of operation (up to 20 days) in pond 1 and pond 2. The bio-film thickness later became constant, being about 30-40 ~m on the lower parts of the carriers under the water and 70-100 ~m on the upper parts of carriers near the surface of pond water. Owing to the possible SS attachment to the AGM, a ~small increase in bio-film thickness had been observed near the inlet of pond system. After some periods of time, the film reached such a thickness that the substrate was utilized before penetrating the entire depth of the microbial film. The microorganisms that existed in the starvation zone would utilize their own cytoplasmic material to maintain life-support functions, i.e. they were in the endogenous growth phase. In
4 9
/~ ~l 5
6 ! ! ! ! I
! ! ! ! !
243
t
-/
(a) Plain layout
.~.
(b) Vertical section Fig. 1. AGP system in series. 1. AOP l(fl~ = 22%). 2. AGP 2(fl2= 22%). 3. Pond 3(fl3= 0). 4. Wastewater tank (2600 x 900 x 500). 5. Submersible pump. 6. Inspection shaft. 7. Influent pipes (D = 25 mm). 8. Effluent pipes (D = 25 ram). 9. Control valves. 10. Artificial SB-type fibrous carriers. 11. Connection pipes (D = 25 ram).
244
Research Note
addition, there existed external shear forces around bio-film caused by wind, rainfall and water flow velocity variation. Therefore, the microorganisms in this phase lost the ability to cling to the fibrous support materials and detached themselves from the supports and settle down to the pond bottom. Because there are great difficulties to describe microbe quantatively by using bio-film thickness, biomass is usually adopted for this purpose. Biomass can be obtained by putting some other bunches of AGM into ponds and taking a bunch of AGM out of pond in a definite period of time, by washing them and weighing the detached mass both in wet and dry conditions. It could be known from these tests that biomass increased progressively up to 15-30 days, then became constant. The volatiles within the biofilm formed on AGM in steady state are 80.0-86.5% in pond 1 and 83.1-91.3% in pond 2 respectively, which will represent the quantities of biomass. As is known to all, bacteria are the main decomposer of organic pollutants in the conventional suspended-growth ponds and play a vital role in the natural stabilization processes, being widely utilized for the treatment of organic wastewaters. They are also the dominant species in AGP system, which may also illustrate the bio-film activities on some degree. The experimental results of bacteria examination are 5 x 105 MPN/gAGM(wet) in pond 1 and 9 x 104MPN/gAGM (wet) in pond 2 respectively, and with the vertical distribution of larger amounts on AGM near the water surface. In addition to large quantities of bacteria attached on the AGM, a vast species of algae and protozoa as well as metazoa or some higher animals are also attached and grown on the artificial fibrous carriers to form various food chains. It can be known from the microscope-examined results of algae and protozoa that algal counts are higher in pond 2 than those in pond 1, except for Euglena. Of the 15 algal species, Oscillatoria, Euglena, Chlorella and Nitzschia are the dominant species in both pond 1 and pond 2. They and any other algae presented in the AGM utilize the end-products such as CO2 and NH4-N of bacterial metabolism from attached- or suspendedgrowth phases, synthetize algal cells and release oxygen to the pond water. Thus every slice of fibrous carriers, especially those near the water surface, serve as small aerators in the day-time. This is the reason that so many oxygen bubbles appear on the surface of AGM. With regard to protozoa and metazoa there are quite abundant of microbe such as Vorticella and Epistylis in the attached fibrous carriers, which may not appear in the conventional suspended-growth ponds. Of greatest interests is that there are also large amount of higher animals such as Rotifers, Roundworms and Daphnia which also play an important role in purification of wastewater, because Rotifers can effectively consume dispersed or coagulated bacteria and small granual organic matters to make effluent clarified, and they usually serve as indicators showing
quite effective aerobic treatment of wastewater having been realized. Roundworms are the main predators of bacteria and protozoa rather than soluable organic materials, which appear only in the water environment of enough dissolved oxygen. Bio-film formed in the attached-growth media is the ideal food sources to Roundworms, by which bio-film become loose, detaches from support media and renews continuously so that oxygen transfer increases and film has vigourous activity. Daphnia is a minute crustacean which predates mainly small particles such as bacteria, single-cell algae, protozoa and the decayed organic materials. When there are enough food sources and DO, it proliferates and grows quite rapidly in the temperature of 15-25°C. From the statements above, both Roundworms and Daphnia are the indicators to show that very low organic pollutants and stable water quality, high DO level and effective treatment of domestic wastewater by such a AGP system.
Benthal deposit As in the conventional WSP, the sludge accumulated in the AGP system is also involved in the overall treatment process due to the detachment of biofilm from AGM and to the anaerobic digestion of sedimented organic matters as well as trapping of undegradable solids. In order to know some information about the benthal deposit in AGP system, five culture vessels (D = 105 mm) were equidistantly put into pond 1 and pond 2 near the bottoms on July 20 respectively, hang-taken out and analysed, which gave the results that sludge accumulation rate ranged from 4.62 to 6.58 mm in pond 1 and from 3.10 to 4.48 mm in pond 2 monthly. The values of deposit depth and other parameters for pond 1 were some greater than those for pond 2, which was simply because of carryover of inorganic solids from influent of domestic wastewater. Volative percentages in the deposit for pond 2 are greater than those for pond 1, which may imply that organic matters or detached bio-film from AGM or dead microorganisms as algae deposited in the subsequent pond.
pH Values and DO levels As routine analyses, pH and dissolved oxygen are monitored almost every day. While hourly monitoring results of typical pH and DO content, conducted on August 5 for the AGP system, had shown that variation of DO is typical of that usually expected in the cyclical increase and decrease during day and night periods for all the three ponds, respectively. DO levels increase from pond 1 to pond 3 progressively, which may be contributed to the organic pollution becomes lighter and lighter, and maximum DO concentrations in pond 3 reaches about two times the sturation level (above 20 mg/l). With regard to pH variation, the rise in DO level is accompanied consistently by a simultaneous increase in pH. In general, increased algal activities are accompanied by in-
Research Note creased pH during day hours, which reaches 7.85 in pond 1, reaches 8.40 in pond 2 and 9.30 in pond 3, respectively.
245
authors wish to express their thanks to Ms Shumei Chen for her efforts to the analytical works and technical assistance during the study. Acknowledgement--The
COD, B O D s and N H 4 - N reduction f r o m A G M system
The purpose of incorporating AGM into ponds is the maximum reduction of organic pollutants in terms of COD, BOD5 and NH4-N reduction. It has been found from the experimental results that COD and BOD5 removal in such a AGP system are quite satisfactory. According to the statistical data from analyses, COD and BOD 5 removals in the AGP system are 75.6% (range from 50% in winter to 91.4% in summer) and 90.2% (range from 49% in winter to 95.9% in summer) respectively. With regard to NH4-N, its removal can be expected from 30.3% to 95.9%, dependent also upon temperature. These correspond with the experimental results from a laboratory studies (Zhao, 1991).
CONCLUSION The incorporation of attached-growth media (AGM), or artificial fibrous carriers used here in the study, enhanced the performance of conventional waste stabilization ponds by the formation of a great number of small stable ecological systems around AGM, being abundant of biospecies from bacteria and algae to protozoa and higher animals, increasing the biomass, improving the biological distribution, which have been confirmed by the study of pilot-scale AGM system. The experimental results also illustrate that satisfactory effluent quality can be expected in such a AGP system, although the total detention time of the system has been controlled and shortened to only 7.5 days during operation.
REFERENCES
APHA (1980) Standard Methods for the Examination of Water and Wastewater, 15th edn. Am. Pub. Health Assoc., Washington, D.C. Ellis K. V. (1990) Trends in wastewater treatment. Developing Worm Water. Grosvenor Press International. NEPA (1989) Anatytical Methods for Water and Wastewater, 3rd edn. Chinese Environmental Publishing House, Beijing (in Chinese). Parker D. S. (1988) Wastewater technology innovation for the year 2000. J. environ. Engng, Am. Soc. Cir. Engrs 114, 487 505. Polprasert C. and Charnpratheep K. (1989) Heavy metal removal in attached-growth waste stabilization ponds. Wat. Res. 23, 625-631. Qi P. S., Wang B. Z., Ma F., Zhang J.S. and Li T. J. (1993) Intensification of a pond by fibrous carriers. Wat. Sci. Technol. 28(7), 117-123. Shin H. K. and Polprasert C. (1987) Attached-growth waste stablization pond treatment evaluation. Wat. Sci. Technol. 19(12), 229-235. Shin H. K. and Polprasert C. (1988) Ammonia nitrogen removal in attached-growth Ponds. J. Environ. Engng, Am. Soc. Cir. Engrs 114, 846 863. U.S. Environmental Protection Agency (1983) Design Manual--Municipal
Wastewater
Stabilization
Ponds.
EPA625/I-83-105, Cincinnati, Ohio. Vuillot M. and Boutin C. (1987) Waste stabilization ponds in Europe~a state of art review. Wat. Sci. Technol. 19 (12), 1-6. Wang B.Z. (1990) Water Pollution Control Engineering. High Education Press, Beijing (in Chinese). Wang B. Z. (1991) Ecological waste treatment and utilization system--a low-cost, energy-saving/generating and resources recoverable technology for water pollution control in China. Wat. Sci. Technol. 24(5), 8-19. Zhao Q. L. and Zhang Z. J. (1991) Temperature influence on performance of oxidation ponds. Wat. Sci. Technol. 24(5), 85-96.