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.     

Flocculation of activated sludge flocs by stimulation of the aerobic biological activity

Activated sludge flocs are known to deflocculate under short-term anaerobic conditions, but little is known about possible reflocculation under subsequent aerobic conditions. When activated sludge flocs from two wastewater treatment plants deflocculated under anaerobic conditions with well-defined shear conditions, they could be almost, but not completely, reflocculated by aeration for 1-2 h under the same shear conditions. If the biological activity was reduced by adding azide, chloramphenicol or by decreasing the temperature, no or only very little reflocculation took place. This indicated that the reflocculation was under direct or indirect microbial control. Only a small part of the reflocculation was due to improved flocculation properties obtained by oxidation of Fe(II) to Fe(III), which is a better flocculant. Fe(II) was produced under the anaerobic conditions by microbial iron reduction, and it was oxidized to Fe(III) within less than one hour after the aeration was started. However, by comparing two different sludges with different capabilities for iron reduction, iron oxidation and responses to substrate addition, it was found that the aerobic biological activity most likely was of greatest significance for the observed reflocculation and floc formation under aerobic conditions. This was further supported by adding organic substrates (glucose or ethanol) during the aerobic reflocculation phase, which promoted reflocculation. However, some substrates had the opposite effect (acetate and lactate), where a deterioration of the reflocculation was observed, probably due to different responses from different groups of microorganisms in the sludges.     

厌氧消化污泥脱水性能变化的试验与分析

研究了剩余污泥在中温厌氧消化条件下脱水性能的变化及其作用机制。剩余污泥厌氧消化过程中,消化污泥的比阻(SRF)相比于剩余污泥有一定的减小,消化污泥的过滤速度有一定的改善,但改善不明显。聚丙烯酰胺(PAM)、FeCl3和聚合氯化铝(PAC)3种絮凝剂调理试验显示,消化污泥的最佳投药量相对于剩余污泥均有所增加,说明消化污泥脱水性能变差。分析了2种污泥中胞外聚合物(EPS)含量及污泥颗粒特性的变化,表明消化过程导致EPS的降解并向液体中释放。随着EPS含量的减少,由EPS架桥形成的较大絮体解体成为较小的污泥颗粒,污泥中小颗粒的比例增加,污泥的脱水性能变差。     

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.     

New generic mathematical model for WWTP sludge digesters operating under aerobic and anaerobic conditions: Model building and experimental verification

This paper presents a new mathematical model developed to reproduce the performance of a generic sludge digester working either under aerobic or anaerobic operational conditions. The digester has been modelled as two completely mixed tanks associated with gaseous and liquid volumes. The conversion model has been developed based on a plant wide modelling methodology (PWM) and comprises biochemical transformations, physicochemical reactions and thermodynamic considerations. The model predicts the reactor temperature and the temporary evolution of an extensive vector of model components which are completely defined in terms of elemental mass fractions (C, H, O, N and P) and charge density. Thus, the comprehensive definition of the model components guarantees the continuity of elemental mass and charge in all the model transformations and between any two systems defined by the model. The aim of the generic digester model is to overcome the problems that arise when trying to connect aerobic and anaerobic digestion processes working in series or to connect water and sludge lines in a WWTP. The modelling methodology used has allowed the systematic construction of the biochemical model which acts as an initial illustrative example of an application that has been experimentally verified. The variation of the temperature is also predicted based on a thermal dynamic model. Real data from four different facilities and a straightforward calibration have been used to successfully verify the model predictions in the cases of mesophilic and thermophilic anaerobic digestion as well as autothermal thermophilic aerobic digestion (ATAD). The large amount of data from the full scale ATAD and the anaerobic digestion pilot plants, all of them working under different conditions, has allowed the validation of the model for that case study.     

Evaluation and Development of a Thermophilic Aerobic Digester at Castle Donington Sewage-Treatment Works

An investigation into the practicality of thermophilic aerobic digestion has been in progress at Castle Donington since 1989. The work has been carried out in a glassed steel insulated digester of 123 m³ capacity. Both auto-entraining and blower-assisted venturi systems have been evaluated.
Both aeration systems were able, under favourable conditions, to remove at least 50% of the sludge COD. It was found that the additional flexibility of the blower venturi combination was necessary to accommodate crude sludges varying from 2% to 12% dry solids.
Originally the plant was intended to operate at above 55C with a 10-day retention period. In practice retention periods at this temperature have been at least 15 days. The process has proved relatively susceptible to inhibition and requires the air supply to be closely matched to the load.
The revenue costs have been, at best, around 47.3/tDS. This is competitive with small-scale anaerobic digestion, but not with the large digestion centre approach widely used in the UK. Unless pasteurization becomes a requirement for agricultural disposal, thermophilic aerobic digestion is unlikely to find wide use in this country.     

Biodegradability of activated sludge organics under anaerobic conditions

From an experimental and theoretical investigation of the continuity of activated sludge organic (COD) compounds along the link between the fully aerobic or N removal activated sludge and anaerobic digestion unit operations, it was found that the unbiodegradable particulate organics (i) originating from the influent wastewater and (ii) generated by the activated sludge endogenous process, as determined from response of the activated sludge system, are also unbiodegradable under anaerobic digestion conditions. This means that the activated sludge biodegradable organics that can be anaerobically digested can be calculated from the active fraction of the waste activated sludge based on the widely accepted ordinary heterotrophic organism (OHO) endogenous respiration/death regeneration rates and unbiodegradable fraction. This research shows that the mass balances based steady state and dynamic simulation activated sludge, aerobic digestion and anaerobic digestion models provide internally consistent and externally compatible elements that can be coupled to produce plant wide steady state and dynamic simulation WWTP models.     

臭氧氧化/厌氧消化工艺处理CEPT污泥的研究

上海市白龙港污水处理厂采用化学一级强化处理工艺（CEPT），污泥产量大且稳定性差，直接脱水填埋很难满足有关环保要求，因此对其进行稳定化和减量化研究是污泥处理、处置工作的重中之重。为此，开展了臭氧氧化／厌氧消化工艺处理该类污泥的中试研究。结果表明，当臭氧投量为0．08kg／kgDS、臭氧氧化污泥量占进泥量的比例为67％、污泥投配率为5％时，臭氧氧化对厌氧消化的促进作用比较明显，对有机物的降解率较单纯厌氧消化的高约6．3％，产气量增加了近20．1％。此外，投加臭氧还能改善污泥的脱水性能，并减少了脱水时助凝剂的投量。     

The thickening and treatment of sewage sludges to minimize phosphorus release

The Johannesburg City Council has operated biological and chemical nutrient removal wastewater treatment plants for the past 15–16 years. With the advent of more sophisticated sludge handling techniques, problems have been experienced with the recycle of phosphorus-rich sludge processing liquors to the liquid phase. These problems are expected to increase in the future. This paper describes laboratory and pilot scale experiments to determine viable methods for phosphorus removal from sludge liquors and ways of obviating phosphorus release from phosphorus-rich activated sludges. The design of new full-scale plants being constructed to pretreat phosphorus-rich sludges prior to dewatering are also discussed. The paper also describes innovative process layouts for sludge treatment keeping in mind various options for final sludge disposal. Emphasis is placed on the pre-aeration and phosphate precipitation from anaerobically digested and raw primary sludges and techniques for the thickening and dewatering of primary and waste activated sludges to minimize phosphate release into the liquors so produced. The technique of sludge thickening on linear screens is described and the coupling of linear screens to belt presses to complete the dewatering operation.     

Sequential polymer dosing for effective dewatering of ATAD sludges

Dewatering problems associated with the sludge from autothermal thermophilic aerobic digestion (ATAD) of sludge, result in large chemical conditioning costs for effective dewatering. A variety of chemical conditioners were used to improve dewatering, but none of them were able to dewater the sludge as desired at acceptable conditioning doses. It was found that during the digestion process, chemical precipitation of divalent cations occurred. Some of the ATAD sludge colloids were found to have a positive zeta potential and these were thought to be the precipitated divalent cations. Sequential polymer dosing using either iron or cationic polymer, followed by anionic polymer, was found to improve dewatering. The use of anionic polymer is essential and allows the use of smaller amounts of iron or cationic polymer for effective dewatering. The use of the less expensive anionic polymer along with cationic polymers has the potential to make the use of the ATAD process more economical.     

THE EFFECTS OF IMPROVING SEWAGE SLUDGE DIGESTION

A model was developed and tested to investigate the effects of various parameters on the processing costs of anaerobic digestion. The model was based on empirical data collected from full-scale plants which varied in size from 35,000 to 900,000 population equivalents. (Equivalent to loading rates of 0.5 to 2.7 kg VS/m³/d.) In spite of different operating regimes and conditions of the plants analysed, various relationships were discovered between: sludge quality and volatile solids destruction; sludge quality and biogas yield; and, effluent volatile solids and dewatering. Where correlations existed, these were used to make predictions on operating costs of the digestion and dewatering plant based on sensitivity analysis. The results of the model were used to make recommendations on how to optimise the operation of an anaerobic digestion plant.     

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.     

Enhancement of anaerobic batch digestion of sisal pulp waste by mesophilic aerobic pre-treatment

Pre-treatment of sisal pulp prior to its anaerobic digestion was investigated using an activated sludge mixed culture under aerobic conditions in batch bioreactors at 37 degrees C. The progression of aerobic pre-treatment of the residue in relation to the activities of some extracellular hydrolytic enzymes in the slurry was monitored. The highest activity of hydrolytic enzymes was obtained at 9 h of pre-treatment. Filter paper cellulase had a maximum activity of 0.90 IU/ml, while carboxymethyl cellulase, amylase and xylanase were produced to a maximum of about 0.40 IU/ml. The methane yield obtained after anaerobic digestion of the pre-treated pulp ranged between 0.12 and 0.24 m3 CH4/kg VS added. The highest and lowest values were obtained for 9 and 72 h of pre-treatment, respectively. Nine hours of pre-treatment of sisal pulp prior to anaerobic digestion demonstrated a 26% higher methane yield when compared to the sisal pulp without pre-treatment. The consortia of microorganisms in activated sludge demonstrated a useful potential in the production of hydrolases acting on major macromolecules of sisal pulp. The fact that a correlation was observed between high enzyme activity and high methane yield at 9 h of aerobic pre-treatment suggests that such a short pre-treatment period could be an alternative option for increasing solubilization of sisal pulp and promoting methane productivity.     

城市污水处理厂污泥的好氧处理研究

为了实现污泥的稳定化、减量化和无害化,对城市污水处理厂的浓缩污泥和厌氧消化污泥进行了好氧处理试验研究.试验结果表明,污泥好氧处理使污泥中有机物质含量降低,臭味减小.厌氧污泥经好氧消化处理后,一方面使污泥得到稳定化和减量化;另一方面,可以把污泥中的致病菌杀死.污泥经好氧消化后沉降性能有明显提高,絮凝剂的加入使其脱水性能也得到一定的改善.     

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.     

Characterization of activated sludge exocellular polymers using several cation-associated extraction methods

Evaluation of prior research and preliminary investigations in our laboratory led to the development of an extraction strategy that can be used to target different cations in activated sludge floc and extract their associated extracellular polymeric substances (EPS). The methods we used were the cation exchange resin (CER) procedure, base extraction, and sulfide addition to extract EPS linked with divalent cations, Al, and Fe, respectively. A comparison of sludge cations before and after CER extraction revealed that most of Ca(2+) and Mg(2+) were removed while Fe and Al remained intact, suggesting that this method is highly selective for Ca(2+) and Mg(2+)-bound EPS. The correlation between sludge Fe and sulfide-extracted EPS was indicative of selectivity of this method for Fe-bound EPS. The base extraction was less specific than the other methods but it was the method releasing the largest amount of Al into the extract, indicating that the method extracted Al-bound EPS. Concomitantly, the composition of extracted EPS and the amino acid composition differed for the three methods, indicating that EPS associated with different metals were not the same. The change in EPS following anaerobic and aerobic digestion was also characterized by the three extraction methods. CER-extracted EPS were reduced after aerobic digestion while they changed little by anaerobic digestion. On the other hand, anaerobic digestion was associated with the decrease in sulfide-extracted EPS. These results suggest that different types of cation-EPS binding mechanisms exist in activated sludge and that each cation-associated EPS fraction imparts unique digestion characteristics to activated sludge.     

Thermal pre-treatment of aerobic granular sludge: impact on anaerobic biodegradability

The aerobic granular systems are a good alternative to the conventional activated sludge (AS) ones to reduce the production of sludge generated in wastewater treatment plants (WWTP). Although the quantity of produced sludge is low its post-treatment is still necessary. In the present work the application of the anaerobic digestion combined with a thermal pre-treatment was studied to treat two different aerobic granular biomasses: one from a reactor fed with pig manure (G1) and another from a reactor fed with a synthetic medium to simulate an urban wastewater (G2). The results obtained with the untreated aerobic granular biomasses showed that their anaerobic biodegradability (BD) (33% for G1 and 49% for G2) was similar to that obtained for an activated sludge (30-50%) and demonstrate the feasibility of their anaerobic digestion. The thermal pre-treatment before the anaerobic digestion was proposed as a good option to enhance the BD when this was initially low (33% G1) with an enhancement between 20% at 60 °C and 88% at 170 °C with respect to the untreated sludge. However when the initial BD was higher (49% G2) the thermal pre-treatment produced a slight improvement in the methane production (14% and 18%) and at high temperatures (190 and 210 °C) which did not justify the application of such a treatment.     

Performance of aerobic digestion at different sludge solid levels and operation patterns

Laboratory batch and semi-continuous experiments were carried out at 20°C on the aerobic digestion of primary sludges and waste secondary activated sludges. Solids concentrations up to 60.000 mg l⁻¹ (6%) for primary sludges and 20.000 mg l⁻¹ (2%) for waste secondary activated sludges, were employed. It was found that aerobic digestion of more concentrated sludges was associated with slower solids degradation rates, but in the studied range of concentrations, more concentrated sludges yielded a higher mass of volatile suspended solids decomposed per unit digester volume per day. For the primary and secondary sludges with the same volatile suspended solids concentration, the amounts of volatile suspended solids decomposed per unit digester volume per day were closely comparable.The results from batch digestion experiments proved not to be directly applicable to the design of continuous flow aerobic digesters. It was also demonstrated that the treatment of a sludge with a higher solids concentration could be an effective means of utilizing the digester volume while obtaining a reasonably well stabilized product sludge.     

EFFECTIVE DESIGN OF GAS-MIXING SYSTEMS FOR THICK SLUDGE DIGESTION: TWO CASE STUDIES

In the UK and elsewhere, advances in the design of anaerobic mesophilic digesters have resulted in a trend towards the digestion of thicker sludges. This includes a number of new facilities using 'bolt-on' technologies such as thermal hydrolysis, pre-pasteurisation and enzymic hydrolysis. Thermal hydrolysis utilises thickened sludge, and the digester is fed with a high dry-solids content (typically 12%), thereby intensifying the digestion process and reducing reactor volumes. However, there is limited experience in the design of digester mixing systems for such applications. As the trend continues towards advanced digestion technology, more emphasis will be placed on the basic unit process operation of mixing.
This paper describes the design of a gas-mixing system for thick feed sludges for two full-scale plants at Mogden and Aberdeen. Mogden is a refurbishment project and Aberdeen is a newly constructed plant.     

Biodegradation of the endogenous residue of activated sludge

This study evaluated the potential biodegradability of the endogenous residue in activated sludge subjected to batch digestion under either non-aerated or alternating aerated and non-aerated conditions. Mixed liquor for the tests was generated in a 200 L pilot-scale aerobic membrane bioreactor (MBR) operated at a 5.2 days SRT. The MBR system was fed a soluble and completely biodegradable synthetic influent composed of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic materials, allowed to generate sludge composed of essentially two fractions: a heterotrophic biomass X_H and an endogenous residue X_E, the nitrifying biomass being negligible (less than 2%). The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X_H and X_E: 68% and 32% were obtained, respectively, at a 5.2 days SRT. To assess the biodegradability of X_E, two batch digestion units operated at 35 °C were run for 90 days using thickened sludge from the MBR system. In the first unit, anaerobic conditions were maintained while in the second unit, alternating aerated and non-aerated conditions were applied. Data for both units showed apparent partial biodegradation of the endogenous residue. Modeling the batch tests indicated endogenous residue decay rates of 0.005 d⁻¹ and 0.012 d⁻¹ for the anaerobic unit and the alternating aerated and non-aerated conditions, respectively.