Feasibility of using a chlorination step to reduce excess sludge in activated sludge process

The ultimate disposal of excess sludge generated from activated sludge processes has been one of the most challenging problems for wastewater treatment utilities. Previous work has shown that excess sludge can be minimized successfully by using sludge ozonation to dissolve it into substrates to be oxidized in the aeration tank. However, this approach is a costly option. Therefore, as an alternative solution, we propose to use chlorination to replace ozonation in excess sludge minimization in the light of operational cost. To investigate the feasibility of this low cost approach, this paper mainly focuses on the effect of chlorination on sludge reduction rate, formation of trihalomethanes, sludge settleability, and effluent quality. Two identical activated sludge membrane bioreactors were continuously operated with synthetic wastewater under the same operation conditions for several months. During this period, one pilot unit was used as the reference system without chlorination of excess sludge, while another served as a testing unit, where excess sludge was taken out for conducting chlorination at a dose of 133mg/g MLSS every day and the chlorinated liquor was then returned to the aeration tank. The sludge production rate and the water quality of both the units were analyzed daily. It was observed that the sludge production could readily be reduced by 65% once the chlorination treatment was involved. However, the chlorination treatment also resulted in poor sludge settleability as well as significant increase of soluble chemical oxygen demand in the effluent, which creates potential difficulties in the operation of a conventional treatment plant with gravity clarifiers. However, it has been demonstrated that by integrating the immersed membrane into the activated sludge process these difficulties can be overcome effectively.     

Oxygen uptake studies on various sludges adapted to a waste containing chloro-, nitro- and amino-substituted xenobiotics

Although containing very low concentrations of organics, triaminotrinitrobenzene (TATB) effluent still appeared toxic in shake-flask experiments. Few toxicity effects showed in model activated-sludge plants, provided that these contained suitably adapted organisms, and were run on phenolic waste or phenol as a basic carboniferous load. Oxygen uptake studies indicated that the metabolic processes within the sludge population appeared unusual, and that degradation of TATB effluent required a sludge which was specially adapted.     

Stable Foams and Sludge Bulking: The Largest Remaining Problems (Abridged)

The separation of activated sludge in secondary settling tanks is the crucial step in biological wastewater treatment from the point of view of both final effluent quality and operation of the aeration plant. The sludge-separation problems of bulking and foaming are connected with an excessive growth of filamentous micro-organisms in the biocenosis of activated sludge.
This paper (a) describes the methods which are used for the quantification of activated-sludge separation problems, and (b) summarizes the design recommendations for bulking and foaming control.     

SEQUENCING BATCH REACTORS: A REVIEW

Sewage treatment using the activated-sludge process evolved from the fill-and-draw system which was developed by Ardern and Lockett in 1914; the sequencing batch reactor is a modern development of this system. Its success is due to modern technology, particularly the programmable logic controller which controls operational sequences. The process has a single rectangular reaction tank where the various sequences of wastewater fill and aeration, activated-sludge settlement and effluent decanting are carried out.
This paper reviews the sequencing batch reactor compared with conventional activated sludge, and explains (a) the similarities and differences, (b) the circumstances in which the system is likely to be cost competitive, and (c) modifications that could increase the effectiveness of sequencing batch- reactor systems in the future.     

Aspects of the Behaviour of Filamentous Microbes in Activated Sludge

Filamentous bacteria tend to be an integral part of the activated-sludge floc, and they can impart characteristics to the sludge which have an adverse effect on the sludge properties, resulting in poor settlement or the formation of stable foams. Both can result in the discharge of excess solids in the final effluent. This paper examines (a) the different types of filament in activated sludge, (b) the problems which they cause, and (c) the options which may be used to reduce the impact of the problems. In particular, it examines the problems associated with the species Microthrix parvicella, the use of selectors and the interactions which can occur in sludges dominated by this species.     

不同分子质量区间腐殖酸的氯化反应特性研究

以自配的腐殖酸水样为研究对象,投加次氯酸钠对其进行消毒,考察了消毒过程中三卤甲烷、卤乙酸的生成量及DOC、UV254、A410、A465/A656的变化情况。结果表明,分子质量为50—100ku的腐殖酸为生成三卤甲烷和卤乙酸的最主要前体物;消毒反应后,各分子质量区间腐殖酸的DOC、UV254和A410都有不同程度的下降,且对小分子质量腐殖酸的DOC、UV254和A410的去除率高于对大分子质量腐殖酸的;A465／A656的变化表明消毒过程中腐殖酸的分子质量呈下降趋势。     

Activated Sludge: Latest Developments and a Look Into the Future (Abridged)

This paper examines and discusses (a) the innovations of the activated-sludge process which have been developed in recent years - particularly biological nutrient-removal processes, (b) modifications to minimize the construction costs of activated-sludge systems while simultaneously incorporating biological nutrient-removal techniques, (c) innovative computer-interfaced control methodologies, and (d) proposed methods of improving the biochemical versatility of activated sludge. The paper also presents potential methods of utilizing activated-sludge processes to produce valuable by-products from wastewaters.     

Statistical Analysis of the Performance of a Biological Nutrient-Removal Plant

Thames Water operated a demonstration-scale biological nutrient-removal plant for two years to investigate the feasibility of retrofitting it into existing activated-sludge plants. The plant was operated with and without the addition of short-chain fatty acids to the anaerobic zone. The results from the trial showed that phosphorus removal was possible, but that performance was not consistently good. The data show that the key parameters which affect the effluent soluble phosphorus concentration are (i) the soluble phosphorus and oxidised nitrogen concentrations at the end of the aeration lane, and (ii) the sludge-blanket depth. Further analysis of the data suggests that secondary release of phosphorus was marked when the concentration of short-chain fatty acids in the anaerobic zone was low, even if the sludge blanket was shallow and when oxidised nitrogen was present in the return sludge. In contrast, secondary release of phosphorus was limited when the concentration of short-chain fatty acids in the anaerobic zone was high.     

Bacterial population changes in hospital effluent treatment plant in central India

Hospital effluent with its high content of multidrug resistant (MDR) enterobacteria and the presence of enteric pathogens could pose a grave problem for the community. It was planned at our tertiary care hospital in central India to study the population changes at various steps of effluent treatment plant (ETP) like collection, aeration, clarification, liquid sludge, dried sludge, high-pressure filter and treated wastewater. The study included viable bacterial counts, coliform counts, staphylococcal, enterococcal, Pseudomonas and multiple drug resistant (MDR) gram negative bacterial counts in the different stages of ETP. In order to study the distribution of bacteria as free floating in liquid and adherent to suspended particles, enumeration of the bacteria in the filterate and the sediment was also carried out. The effluent input showed 55% of the 8.6 x 10(6)/ml bacteria as coliforms and E. coli which was a typical of fecal flora. The prevalence of MDR coliforms was 0.26%. The substantial reduction (> 3log) was seen for the effluent coming from the clarifier. The bulk of the bacteria in the hospital effluent remains firmly adhered to solid particles; aeration and clarification removes bulk of the bacteria by physical processes like flocculation. The treated liquid effluent still contains sizeable loads of MDR bacteria and inactivation by procedure such as chlorination is required. The bacteria get concentrated in sludge and a greater concentration of chlorine is required for decontamination.     

Intensification of the Activated-Sludge Process (Abridged)

During the last thirty years, attempts have been made to increase the rate of treatment of wastewater in compact activated-sludge systems whilst ensuring (a) an adequate supply of dissolved oxygen, (b) good settleability of activated sludge, and (c) consistently high-quality effluents.
This paper describes two successful systems, i.e. oxygenated activated sludge and the deep-shaft process, which are used to intensify the rate of treatment in relatively compact aeration tanks. Data are presented of full-scale oxygen activated-sludge systems, including Vitox and Unox, together with information on the operation and performance of the deep-shaft process.     

Studies of the halogenation of dissolved organic material during the chlorination of saline cooling waters, using the model compound resorcinol

When resorcinol is chlorinated in seawater, the variety of reaction products formed is controlled by competing equilibria involving chlorination of the resorcinol and conversion of oxidising species to hypobromous acid by exchange with naturally occurring bromide. When bromide is present in a molar excess over added chlorine, chlorination of resorcinol is only favoured under high pH conditions; under more acid conditions however, bromination occurs at the expense of chlorination. Increasing chlorine levels leads to ring cleavage and the production of trihalomethanes (haloforms).     

外压浸入式超滤膜处理排泥水上清液的研究

采用外压浸入式中空纤维超滤膜处理排泥水重力浓缩上清液。试验结果表明,当温度从16℃下降到2℃时,TMP从0．032MPa上升到0．055MPa。排泥水经6h重力浓缩后,上清液浊度为9．99～80．33NTU,CODMn为5．36～18．64mg／L;超滤膜出水浊度为0．08～0．11NTU,CODMn小于3mg／L,颗粒数小于1个／mL。在排泥水上清液水质波动较大情况下,经超滤处理后,出水水质优于常规处理工艺出水,加氯后可直接进入清水池。     

Reaction of chlorine and bromine with humic substance

Soil humic substance was obtained by extraction from leaf mould with water and then subjected to various chlorination/bromination procedures. The total amount (moles) of trihalomethanes (THMs) formed by bromination was larger than that by chlorination, suggesting that bromine is more reactive towards humic substance than is chlorine. Four bromine compounds were detected after chlorination, whereas 12 brominated compounds were formed by bromination after pre-chlorination. This suggests that some chlorinated products subsequently reacted with the bromine, forming brominated compounds.     

The Two-Stage Moving-Bed/Activated-Sludge Process: An Effective Solution for High-Strength Wastes

The paper describes a moving-bed bioreactor which was used as the first stage of a two-stage process to provide an effective and simple means of reducing the biochemical oxygen demand of a wastewater prior to further treatment in a conventional second-stage activated-sludge plant.
At Corby, a scheme was required to (a) meet the requirements of a more stringent effluent consent and (b) provide capacity for further anticipated growth in industrial effluent loads. The original works comprised conventional primary sedimentation tanks, biological filters and humus tanks, followed by an activated-sludge oxidation ditch and further settlement tanks. A pilot plant has been in operation at Corby for two years, and the results were used to design a moving-bed bioreactor to replace the existing filters.
At Pyewipe, a new coastal treatment plant was required and, following the change in status from a 'high natural dispersion area &, a 'normal' programme was initiated to achieve the required completion date. A value-management workshop was held which identified the two-stage moving-bed bioreactor/activated-sludge process, and a pilot plant was operated for eighteen months. A full-scale plant is now under construction and incorporates a number of innovative features.     

氯化有机副产物的毒性及去除

水中腐殖质是水氯化生成有机卤代物的有效前驱物 ,试验表明有机卤代物对人体十分有害 ,吸附、氧化均可有效地去除卤代有机物 ,氯替代消毒剂的研究与使用是解决此问题的较好方法 .     

pH significantly affects removal of trace antibiotics in chlorination of municipal wastewater

The effect of pH on chlorination behaviors of 12 antibiotics, including β-lactams, sulfonamides, fluoroquinolones, tetracyclines, macrolides, and others at environmentally relevant concentrations was systematically examined in the effluent matrix of activated sludge process. The removal of most antibiotics (except cefalexin and tetracycline) significantly depended on pH in the range of 5.5-8.5. The elimination rates of ciprofloxacin, norfloxacin, anhydro-erythromycin, and roxithromycin increased while that of sulfamethoxazole decreased significantly with the increase of pH. Sulfadiazine, ofloxacin, and trimethoprim exhibited the highest reactivity with free available chlorine under the pH of 6-7, 7, and 7.5, respectively. Not only the free available chlorine species (HOCl and OCl⁻), but also the antibiotics species (cationic, neutral and anionic) affected the overall reaction rate. Anionic antibiotic species are usually much more reactive (1-3 orders of magnitude greater) than cationic antibiotic species toward free available chlorine. Although OCl⁻ is a weaker oxidant than HOCl, chlorination of sulfadiazine, sulfamethoxazole, ciprofloxacin, norfloxacin, and trimethoprim with OCl⁻ became significant at pH > 7.5. The observed kinetics rate constants calculated from species-specific rate constants could accurately (0.91 < R² < 0.99) predict the antibiotic removal in chlorination of activated sludge effluent with similar DOC and ammonia concentration to this study at a given pH value.     

A theoretical study of factors influencing the microbial population dynamics of the activated-sludge process—I The effects of diurnal variations of sewage and carnivorous ciliated protozoa

Mathematical models and computer simulations have been used in examining the theoretical implications of diurnal variations in sewage flow, bacterial content of the sewage, and sewage substrate concentration on the microbial population dynamics and effluent quality of a completely-mixed activated-sludge plant. Variations in sewage substrate resulted in variations in the concentrations of substrate in the effluent and bacteria in the sludge. Variations in the bacterial content of sewage affected only the bacteria-consuming ciliate populations whereas variations in the flow of sewage affected all populations. When all three sewage parameters were varied simultaneously the effect was cumulative.The effects of carnivorous ciliates preying on bacteria-consuming ciliates have been considered for the first time. At least four types of ciliate predator/prey situations could arise since both carnivore and prey can be either free-swimming or attached forms. In general, when the carnivore was a free-swimming form, oscillations, which may or may not dampen, were obtained; when the carnivore was an attached form, the ciliate prey was washed out of the reactor. However, the predictions of the population dynamics of the organisms also depend to a large extent on the values of the various growth constants used.     

Analysis of selected trace organics in advanced wastewater treatment systems

Effluents through four different pilot tertiary wastewater treatment systems were monitored for selected trace organic compounds. The effects of using ozone, free and combined chlorine residuals in these systems were also studied. Advanced treatment of secondary effluent using various combinations of flocculation (alum and polymer), dual media filtration, and carbon adsorption were evaluated for production/removal of volatile halogenated organics, polynuclear aromatics, chlorinated pesticides, and polychlorinated biphenyls. Gas chromatographic methods were used for the analysis of these different classes of compounds: specific techniques and analytical parameters are described. Salient results included: drastic increases in trihalomethane production using free chlorine residuals: disinfection with combined chlorine species does not produce significant levels of trihalomethanes: approximately 90% reduction in trihalomethane levels by carbon adsorption: absence of detectable quantities of polynuclear aromatics: significant decreases in pesticide and PCB levels by carbon adsorption and chlorination. Statistical dependence of trihalomethane production on soluble COD, suspended solids and chloramine levels was evident from multiple linear regression calculations.     

Improvements to Activated-Sludge Plant Aeration Systems at Hawick and Jedburgh Sewage-Treatment Works

In response to the requirement of the Tweed River Purification Board for a higher standard of effluent, improvements have been carried out to the activated-sludge aeration systems at Hawick and Jed-burgh sewage-treatment works.
This paper describes the electrical and mechanical refurbishment involved, and the performance of the completed systems. Process control was considered to be of particular importance in these projects as the plant receives a high proportion of industrial effluent.     

The Development and Use of Hybrid Aeration Processes

Requirements to improve effluent quality have led to the establishment of better design procedures for activated-sludge plants. The recognition that activated-sludge aeration is responsible for a large proportion of the energy used in sewage treatment has resulted in the specification of energy-efficient aeration systems as part of such design procedures. Fine-bubble diffused-air techniques are potentially more efficient than mechanical surface-aeration systems, but low oxygen-transfer efficiency is often observed in practice under conditions of high aeration tank solids loading. Surface-aeration systems appear to be able to operate at reasonable aeration efficiencies under conditions of high loading, but they are not as effective as diffused-air systems in low-rate processes.
There is evidence that a properly designed combination of the two types of aeration system in the same process (hybrid aeration) can result in optimum energy use and good effluent quality. The paper describes the development of this concept, showing the long-term performance of three full-scale plants utilizing hybrid aeration.