Aerobic digestion of thickened activated sludge: Reaction rate constant determination and process performance

The aims of this work were to select which parameter—total suspended solids (TSS), volatile suspended solids (VSS) or degradable volatile suspended solids (DVSS)—best expressed the kinetics of aerobic digestion of waste secondary activated sludge: to define the relationship between the reaction rate constant and the initial concentration of sludges; to evaluate the effect of aerobic digestion on the filterability of the treated sludge as measured by the capillary suction time (C.S.T.).Batch experiments of aerobic digestion were performed in our laboratory on samples of sludges at a constant temperature of 20 C. The initial concentration of sludges ranged from 2600 to 22,000 mg l⁻¹.TSS was found to be the parameter which most closely fits the kinetic model of aerobic digestion. The reaction rate constant was a linear, inverse function of the initial sludge concentration with TSS and with VSS as a parameter.Sludge filterability is affected by the process of aerobic digestion; a prolonged aeration time results in poorer dewatering; and the more concentrated the sample, the poorer the dewatering.     

Biological oxidation and disinfection of sludge

Aerobic digestion, which is biological oxidation of sludge, produces heat. Enough heat is liberated to raise the temperature of the wet sludge up to the pasteurizing range (60–70°C). The major heat loss from an aerobic digester is the latent heat of vaporization of the water vapor which is carried of in the waste air. The use of concentrated oxygen to replace air minimizes this heat loss and theoretically permits thermophilic operation if the initial sludge solids are greater than about 4 per cent. Pilot scale studies have shown that thermophilic aerobic digestion is practical and can be carried out in less than 24 h. Capital and operating costs are reduced and the sludge is pasteurized without the use of external heat. The process is a form of high-rate wet composting and produces a sludge ideally suited for disposal on agricultural lands.Recent and current research data from U.S. EPA-sponsored studies on aerobic digestion of primary sludge solids at Hollywood, Florida; mixed primary and waste activated sludge solids at Hamilton, Ohio; and waste activated sludge solids at Denver, Colorado, are presented and critiqued in light of present design methodology and constraints. These data were obtained under a variety of operating conditions. Data and discussion are also given on the use of pure oxygen in digestion of thickened waste activated sludges.     

Aerobic stabilization of primary and mixed primary-chemical (alum) sludge

This study was conducted to investigate the feasibility of using aerobic digestion as a method for the stabilization of mixed primary-chemical (alum) sludge from a physical-chemical treatment plant. Aerobic digestion was carried out in “continuous” flow (batch fed once a day) laboratory reactors with detention times ranging from 5 to 35 days held at 7, 12, 18 and 25°C.Temperature was found to have a slightly greater influence on the reduction of volatile suspended solids in primary sludge than in mixed primary-chemical sludge. Nitrification took place in the reactors treating both primary and mixed primary-chemical sludge. The high content of aluminum in the mixed primary-chemical sludge did not inhibit the nitrifiers. The oxygen-uptake rate varied between approximately 5 mg O₂ g⁻¹ VSS h⁻¹ and 1 mg O₂ g⁻¹ VSS h⁻¹ depending on the detention time and the reactor temperature. Adenosine triphosphate content per unit volatile suspended solids indicated a low content of active biomass during aerobic digestion.     

The Application of Cross-flow Microfiltration Technology to the Concentration of Sewage Works Sludge Streams

A LOCALLY DEVELOPED cross-flow microfiltration process using woven fabric tubes was used to concentrate waste activated sludge and anaerobic digested sludge from a conventional sewage works.
Results showed that waste activated sludge could be concentrated from 5 to 50 g/l total solids. The permeate quality was good (0 to 50 mg/l suspended solids), but deteriorated both with time and increasing feed solids concentration.
A cross-flow microfilter was coupled to a pilot scale anaerobic digester and the digester solids were increased from 26 to 55 g/l total solids. The organic loading to the digester was increased from 1.8 to 3.1 kg volatile solids per m³ per day. The solids retention time was held constant at 26 days, while the liquid retention time was decreased from 26 to 14 days. The permeate quality was significantly better than the supernatant liquor from a comparable digester (suspended solids 122 and 570 mg/l, respectively).
Data obtained from the cross-flow microfiltration of waste activated sludge was used to regress for the constants in a mathematical model of steady-state flux.     

Volatile solids reduction in two-phase and conventional anaerobic sludge digestion

Success of two-phase anaerobic systems for primary and secondary sludge treatment has been reported based on both directly and indirectly measured volatile solids (VS) reduction, total gas and methane generation, COD reduction, etc. The objective of this research was to determine whether phase separation increases directly measured VS reduction compared to conventional anaerobic sludge digestion. Two-phase and conventional digesters were operated with sludge feeds from three sources; both 1:1 sludges (primary:waste activated, solids basis) and 100% waste activated sludges (WAS) were studied. The maximum difference between VS reductions in conventional and two-phase systems was about 8.7% with waste activated sludge. The increase in volatile solids reduction in two-phase systems with the 1:1 sludge ranged from 1.9 to 6.0% as compared to conventional systems. This relatively small increase in VS reduction may not be worth the additional cost of operating two-phase digesters at full scale.     

Filtration properties of aerobic stabilized primary and mixed primary-chemical (alum) sludge

The filtration properties of aerobic stabilized primary and mixed primary-chemical (alum) sludges have been studied in terms of capillary suction time (CST), specific resistance to filtration and coefficient of compressibility. The sludges were taken from “continuous” flow laboratory aerobic digesters, with detention times ranging from 5 to 35 days held at 7, 12, 18 and 25°C.Specific resistance to filtration and capillary suction time for both primary and mixed primary-chemical sludge generally improved with increasing detention times in the digester, except for the sludges stabilized at 7°C which remained almost unchanged. Mixed primary-chemical sludge showed better filtration properties than primary sludge alone. A fairly good correlation was found between specific resistance to filtration and CST divided by percentage of suspended solids in the sludge sample. The sludge compressibility increased with increasing detention time in the digester for both types of sludges investigated. Anoxic storage of aerobically stabilized sludges caused an increase in specific resistance to filtration.     

The activated sludge process—IV: Application of the general kinetic model to anoxic-aerobic digestion of waste activated sludge

This paper discusses the application of the general activated sludge model as set out by Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) and extended by Van Haandel et al. (Wat. Res.15, 1135–1152, 1981), to anoxic-aerobic digestion of waste activated sludge. The laboratory scale experimental investigation comprised a 6 day sludge age activated sludge process, the waste sludge from which was fed to a number of digesters operated as follows: single reactor flow-through digesters at 4 or 10 days sludge age (retention times) under aerobic or anoxic-aerobic conditions (with 1.5 and 4 h cycle times) and 3-in-series flow-through aerobic digesters each with 4 days sludge age; all digesters were fed draw-and-fill wise once per day. The general kinetic model simulated accurately all the experimental data without the need to change the values of the kinetic constants. Both theoretical simulations and experimental data indicate that (i) the rate of volatile solids destruction is not affected by the incorporation of anoxic cycles and (ii) the specific denitrification rate constant in a digester is about two-thirds of that in the secondary anoxic reactor of the single sludge activated sludge system; this allows definition of a fourth denitrification rate constant K₄ for the anoxic-aerobic digester with K₄T = 0.046(1.029)^(T-20) mg(NO₃-N) (mgAVSS d)⁻¹, a constant independent of sludge age. An important consequence of (i) and (ii) above is that the denitrification can be integrated readily into the steady state digester model of Marais and Ekama (Wat. SA2, 163–200, 1976) and used for design purposes.     

Investigation of the sludge reduction mechanism in the anaerobic side-stream reactor process using several control biological wastewater treatment processes

To investigate the mechanism of sludge reduction in the anaerobic side-stream reactor (SSR) process, activated sludge with five different sludge reduction schemes were studied side-by-side in the laboratory. These are activated sludge with: 1) aerobic SSR, 2) anaerobic SSR, 3) aerobic digester, 4) anaerobic digester, and 5) no sludge wastage. The system with anaerobic SSR (system #2) was the focus of this study and four other systems served as control processes with different functions and purposes. Both mathematical and experimental approaches were made to determine solids retention time (SRT) and sludge yield for the anaerobic SSR process. The results showed that the anaerobic SSR process produced the lowest solids generation, indicating that sludge organic fractions degraded in this system are larger than other systems that possess only aerobic or anaerobic mode. Among three systems that involved long SRT (system #1, #2, and #5), it was only system #2 that showed stable sludge settling and effluent quality, indicating that efficient sludge reduction in this process occurred along with continuous generation of normal sludge flocs. This observation was further supported by batch anaerobic and aerobic digestion data. Batch digestion on sludges collected after 109 days of operation clearly demonstrated that both anaerobically and aerobically digestible materials were removed in activated sludge with anaerobic SSR. In contrast, sludge reduction in the aerobic SSR process or no wastage system was achieved by removal of mainly aerobically digestible materials. All these results led us to conclude that repeating sludge under both feast/fasting and anaerobic/aerobic conditions (i.e., activated sludge with anaerobic SSR) is necessary to achieve the highest biological solids reduction with normal wastewater treatment performance.     

Suspended biofilm carrier and activated sludge removal of acidic pharmaceuticals

Removal of seven active pharmaceutical substances (ibuprofen, ketoprofen, naproxen, diclofenac, clofibric acid, mefenamic acid, and gemfibrozil) was assessed by batch experiments, with suspended biofilm carriers and activated sludge from several full-scale wastewater treatment plants. A distinct difference between nitrifying activated sludge and suspended biofilm carrier removal of several pharmaceuticals was demonstrated. Biofilm carriers from full-scale nitrifying wastewater treatment plants, demonstrated considerably higher removal rates per unit biomass (i.e. suspended solids for the sludges and attached solids for the carriers) of diclofenac, ketoprofen, gemfibrozil, clofibric acid and mefenamic acid compared to the sludges. Among the target pharmaceuticals, only ibuprofen and naproxen showed similar removal rates per unit biomass for the sludges and biofilm carriers. In contrast to the pharmaceutical removal, the nitrification capacity per unit biomass was lower for the carriers than the sludges, which suggests that neither the nitrite nor the ammonia oxidizing bacteria are primarily responsible for the observed differences in pharmaceutical removal. The low ability of ammonia oxidizing bacteria to degrade or transform the target pharmaceuticals was further demonstrated by the limited pharmaceutical removal in an experiment with continuous nitritation and biofilm carriers from a partial nitritation/anammox sludge liquor treatment process.     

A survey of municipal anaerobic sludge digesters and diagnostic activity assays

A plant site survey was made of 30 municipal wastewater treatment plants concerning the operational characteristics of their anaerobic sludge digesters. Design information, operating data and analytical data were tabulated. Samples of each sludge were then assayed to determine the residual gas production rate, the maximum potential acetate and propionate utilization rates, the 5 and 30 day biochemical methane potential (BMP₅) (BMP₃₀) and the possible limitation in bioavailability of iron, cobalt or nickel.The average solids content of the raw sludge fed to the digesters was 4. 7%. Eight of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and acetate was unlimiting. Nine of the 30 sludges showed stimulation in the gas production rate when iron, cobalt or nickel was supplemented and propionate was unlimiting.The average BMP₅ was 0. 7 volumes of methane per volume of sludge. The average BMP₃₀ was 1.9 vol CH₄/vol sludge. On average 87% of the biodegradable fraction of the sludge was converted to methane during digestion. The average maximum potential acetate utilization rate (MPAUR) was 0.93 vol CH₄/vol sludge-day. The average maximum potential propionate utilization rate (MPPUR) was 0.18 vol CH₄ /vol sludge-day. The average H₂S in the digester gas was 2200 ppm. The average digester capacity was 0.14 million gallons/million gallons per day (MG/MGD) of raw wastewater. On average the raw sludge pumping rate was 4300 gal per MG of raw wastewater. The average digester gas production was 0.066 vol gas/vol of raw wastewater (8800 ft³ per MG). The average unit gas production rate was 0. 64 volumes of digester gas per volume of digester per day. The average volume of digester gas produced per volume of raw sludge feed was 15. 8 v/v. The average gas production per pound of volatile solids added was 0. 46 m³ kg⁻¹ VS (7. 4 ft³ lb⁻¹ VS). The average gas production per pound of volatile solids destroyed was 0. 94 m³ kg⁻¹ VS (15. 0 ft³ lb⁻¹ VS).     

Fate of pathogens in thermophilic aerobic sludge digestion

The effect of autoheated aerobic thermophilic digestion on the pathogen content of sewage sludges was studied and compared to that of conventional mesophilic anaerobic digestion. Both systems were full scale, continuously-fed facilities operated in parallel and utilized a feed sludge of thickened primary and waste-activated sludge.The relative populations of viruses, Salmonella sp., total and fecal coliforms, fecal streptococci and parasites found before and after digestion were compared. The full scale mesophilic anaerobic digesters were operated at relatively constant conditions, i.e. digester temperature constant at 35°C, and loading rates constant, etc., while the full scale autoheated aerobic digester was operated under a wide range of loading conditions. At all of the conditions studied, the autoheated digester temperature exceeded 45 C. Virus and Salmonella sp. concentrations in the effluent from the aerobic unit were below detectable limits in 10 of 11 samples and 6 of 6 samples, respectively, whereas the anaerobic digester effluent contained detectable numbers of viruses and Salmonella sp. Bacterial indicator counts and parasite concentrations were less in the autoheated digester effluent than in the effluent from the anaerobic digester. It was concluded that the simple autoheated aerobic digestion process could be used to produce a virtually pathogen-free sludge at a cost comparable to that of conventional, mesophilic anaerobic digestion.     

The treatment of leachates from domestic wastes in landfills—I: Aerobic biological treatment of a medium-strength leachate

A medium-strength leachate from domestic solid wastes in a landfill (COD 5000 mg l⁻¹, BOD 3000 mg l⁻¹ was treated using aerobic biological processes in continuous-flow, laboratory-scale reactors at low temperatures. Each unit was completely mixed, and mixed liquor was wasted such that solids retention time (SRT) was equal to the hydraulic retention period.At 10 C with addition of phosphate (COD:P less than 100:1) SRT values of 10 days were required to obtained well-clarified effluents, and high removal of BOD (>;98%) and COD (>;92%). Reduction of temperature to 5°C resulted in adverse effects on settling of sludges from units with SRT values of less than 10 days, but in other units good removal of organic materials could still be obtained. These units operated successfully with concentrations of mixed liquor volatile suspended solids (MLVSS) of 1450 mg l⁻¹, equivalent to a ratio of F/M of 0.21 kg BOD kg⁻¹ MLVSS day⁻¹ or less. Removal of ammoniacal nitrogen which took place (influent concentration 80 mg l⁻¹ as N) resulted from incorporation in biomass, and at SRT values of 10 days no nitrification took place at 5 or 10 C. Higher concentrations of ammonia in influent leachates resulted in ammonia in effluents when the ratio of BOD:N was less than about 100:3.6. Increasing the SRT of units to 20 days resulted in erratic conversion to nitrite, but reduced pH-values and possible simultaneous denitrification caused floating sludges and poorly-clarified effluents. Removal of ammonia is identified as a major problem when treating leachates, and further research is recommended.     

Combined anaerobic and aerobic digestion for increased solids reduction and nitrogen removal

A unique sludge digestion system consisting of anaerobic digestion followed by aerobic digestion and then a recycle step where thickened sludge from the aerobic digester was recirculated back to the anaerobic unit was studied to determine the impact on volatile solids (VS) reduction and nitrogen removal. It was found that the combined anaerobic/aerobic/anaerobic (ANA/AER/ANA) system provided 70% VS reduction compared to 50% for conventional mesophilic anaerobic digestion with a 20 day SRT and 62% for combined anaerobic/aerobic (ANA/AER) digestion with a 15 day anaerobic and a 5 day aerobic SRT. Total Kjeldahl nitrogen (TKN) removal for the ANA/AER/ANA system was 70% for sludge wasted from the aerobic unit and 43.7% when wasted from the anaerobic unit. TKN removal was 64.5% for the ANA/AER system.     

Removal of organic acids by activated sludges

Removal of coexisted volatile organic acids was studied using three kinds of activated sludges; treated with sewage, digested night soil and undiluted night soil at the plant scale and laboratrial experiment. Concentration of volatile fatty acids in sewage were too low to be detected, meanwhile 5–28 ppm of acetic acid were detected in influent of aeration tank of digested night soil treatment plant, and 1335–5340 ppm of acetic acid were detected in night soil. Removal rates of acetic acid were 35.9 mg g⁻¹ h⁻¹ by sewage activated sludge, 33.6 mg g⁻¹ h⁻¹ by digested night soil activated sludge and 16.9 mg g⁻¹ h⁻¹ by undiluted night soil activated sludge under coexisting volatile fatty acids. This difference depends on the number of bacteria in the activated sludge. Dissimilation percentage of acetic, propionic, butyric and valeric acids were similar results in these activated sludges.     

Effect of copper on nitrification in activated sludge

The concentration of free copper in activated sludge with copper added is strongly influenced by pH. For example, at pH 6.5 with 9.13 × 10⁻⁵ mol Cu l⁻¹, the free copper concentration is 4.0 × 10⁻⁷ mol l⁻¹ (pCu = 6.4) and at pH 8.4 this concentration is 10⁻⁸ mol l⁻¹ (pCu = 8.0). In both cases the activated sludge concentration is 0.7 g MLSS l⁻¹. The free copper concentration is also affected by the concentration of mixed liquor suspended solids (MLSS).In batch experiments with constant pH, the effect of copper on the nitrification rate was not regulated by total copper concentration but by copper/sludge ratio or by free copper concentrations. Experiments at different pH showed a linear correlation between nitrification capacity and free copper concentration, suggesting that the pH effect on nitrification below 8.3 is in fact a copper effect.Activated sludge with copper did not become acclimatized to the copper in a period of three days. Addition of nitrilotriacetic acid (NTA) within one day did cancel the copper inhibition.The results were compared with the effect of copper on acetate removal by heterotrophic micro-organisms. The nitrifiers proved to be no more susceptible to copper than heterotrophic micro-organisms.     

The behaviour of polychlorinated biphenyls in the primary sedimentation process of sewage treatment: A pilot plant study

The behaviour of polychlorinated biphenyls (PCB) in the primary sedimentation stage of sewage treatment has been studied using a pilot scale primary sedimentation plant. The plant was operated at different hydraulic loadings, selected to cover the range expected during normal operation of a sewage treatment works. Concentrations of PCB in raw sewage were found to be in the low ng l^?1 range and in the high ng l^?1 range in primary sludges. A relationship between raw sewage PCB concentrations and variations in the raw sewage flow to the full-scale sewage treatment works was evident. Removal efficiencies of PCB were found to be highest at dry weather flow and lowest at three times dry weather flow, their removals being comparable to those of suspended solids. Mass balances of PCB and suspended solids in the treatment process have been calculated and the possible sources of error considered. Comparisons of the results have been made with those of previous studies. The implications of the presence of PCB in waste waters and sewage sludges for water re-use and sewage sludge disposal are discussed.     

Slow sand filtration as a technique for the tertiary treatment of municipal sewages

The investigation was designed to demonstrate the viability, or otherwise, of slow sand filtration as a means of tertiary treatment for secondary effluents derived from conventional aerobic, biological treatment processes operating with municipal wastewaters. Secondary effluents derived from both an activated-sludge plant and from a percolating filtration plant were employed.The basic slow sand filtration unit used consisted of a 140 mm i.d. perspex cylinder, 2.65 m in height containing a 950 mm depth of fine sand. Treatment rates were either 3.5 or 7.0 m d⁻¹ and the sand used was of an effective size initially of 0.3 mm and then later of 0.6 mm.This investigation has demonstrated that a laboratory-scale slow sand filtration unit is capable of consistently removing at least 90% of the suspended solids, more than 65% of the remaining BOD and over 95% of the coliform organisms from the settled effluent from an operational percolating filter plant. The length of operational run averaged 20 days at 3.5 m d⁻¹ and 13 days at 7.0 m d⁻¹. Slightly inferior results were achieved when using the settled effluent from an operational activated sludge unit.Further investigation employing a horizontal-flow gravel pre-filter demonstrated that at flows of 2 m h⁻¹ with a contact time of 33 min up to 82% of the suspended solids in the secondary effluent could be removed prior even to slow sand filtration.     

Tracking the inorganic suspended solids through biological treatment units of wastewater treatment plants

From an experimental and theoretical investigation of the continuity of influent inorganic suspended solids (ISS) along the links connecting the primary settling tank (PST), fully aerobic or N removal activated sludge (AS) and anaerobic and aerobic digestion (AerD) unit operations, it was found that (i) the influent wastewater (fixed) ISS concentration is conserved through primary sludge anaerobic digestion, and AS and AerD unit operations. However, the measured ISS flux at different stages through a series of WWTP unit operations is not equal to the influent ISS flux because the ordinary heterotrophic organisms (OHO) biomass contributes to the ISS flux by differing amounts depending on the OHO (active) fraction of the VSS solids at that stage.     

Thermophilic anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge

Anaerobic digestion of pulp and paper mill primary sludge and co-digestion of primary and secondary sludge were studied for the first time in semi-continuously fed continuously stirred tank reactors (CSTR) in thermophilic conditions. Additionally, in batch experiments, methane potentials of 210 and 230 m³CH₄/t volatile solids (VS)_added were obtained for primary, and 50 and 100 m³CH₄/tVS_added for secondary sludge at 35 °C and 55 °C, respectively. Anaerobic digestion of primary sludge was shown to be feasible with organic loading rates (OLR) of 1-1.4 kgVS/m³d and hydraulic retention times (HRT) of 16-32 d resulting in methane yields of 190-240 m³CH₄/tVS_fed. Also the highest tested OLR of 2 kgVS/m³d and the shortest HRT of 14-16 d could be feasible, if pH stability is confirmed. Co-digestion of primary and secondary sludge with an OLR of 1 kgVS/m³d and HRTs of 25-31 d resulted in methane yields of 150-170 m³CH₄/tVS_fed. In the digestion processes, cellulose and hemicellulose degraded while lignin did not. pH adjustment and nitrogen deficiency needs to be considered when planning anaerobic digestion of pulp and paper mill wastewater sludges.     

The anaerobic digestion of waste from an intensive pig unit

Anaerobic digestion of piggery wastes as a primary method of purification was studied in laboratory-scale (15 l.) heated, stirred digesters. It was impossible to obtain a balanced digestion by fermentating undiluted faeces-urine slurry, but balanced fermentations could be built up in digesters originally seeded from a working domestic anaerobic digester or in digesters filled with water to which small amounts of waste were regularly added. The results from running two digesters for over 80 weeks at loading rates from 0·03 to 0·20 lb VSS ft^?3 capacity day^?1 are given, and it is concluded that the anaerobic digestion of piggery wastes is possible and that improvement in the waste is obtained, but that there may be an upper limit to the solids content of the digester input. A secondary aerobic treatment of the settled anaerobic digester output improves the liquid.