Low temperature removal of nitrate by bacterial denitrification

The efficiency of two denitrifying sludges enriched at 5 and 20°C were compared using methanol as an electron donor. Both sludges were exposed to the same hydraulic and chemical conditions using an influent containing methanol and mineral salts. The low temperature sludge seemed to have several advantages over the sludge selected for at the higher temperature. In the range 0–17°C, the specific denitrification rate was 1.5–4 times the rate for the high temperature sludge, temperatures below 8°C being the most favourable. At 2°C, under nitrate limiting conditions, 98% nitrate reduction was obtained at a hydraulic residence time of 3.5 h, with an effluent concentration of 0.8 mg NO₃---Nl⁻¹. Sedimentation characteristics were always better for the low temperature sludge, and the utilization of methanol equally good as the high temperature sludge. The low temperature sludge appeared to be biochemically and microbiologically stable to temperature changes within the range 0–17°C, the latter temperature being close to the limit for maintaining the psychrophilic characteristics of the sludge. Studies on pure culture isolates of the denitrifying bacteria showed >90% dominance of one bacterial strain in both sludges. Studies of the isolates also showed that the low-temperature sludge consisted predominantly of psychrotrophs/psychrophiles, and not well-adapted mesophiles, which were only present in low concentrations. The dominant strain in both sludges was unable to grow on methanol in pure culture without access to nutrient growth factors. Only a few minor strains were obligate methylotrophs.Low temperature sludges were tested in a 3-stage biological process receiving domestic sewage. Each stage; carbon oxidation, nitrification and denitrification had separate sludge recycle, and methanol was added to the denitrification stage. These sludges were grown and selected for at temperatures 5°C. At 5°C the laboratory scale process gave 90% removal of total nitrogen at hydraulic residence times of 1.5, 9 and 4 h for the two aeration stages and the anaerobic stage respectively. Overall nitrification/denitrification was 95%, while denitrification separately was 98%. The effluent contained 0.4 mg NO₃---Nl⁻¹. The critical step in the process was unquestionably nitrification. Oxidation of ammonium was satisfactory at low temperature, but the reaction was somewhat vulnerable to changes in external conditions. The low temperature denitrifying sludge was originally enriched on synthetic waste but did not appear to change its microbial composition or characteristics by exposure to municipal wastewater.     

Nitrification kinetics in activated sludge at various temperatures and dissolved oxygen concentrations

Activated sludge from a domestic sewage works was enriched with nitrifying bacteria by running a laboratory fermenter on ammonia-supplemented sewage. This enriched culture was used to determine respirometrically the kinetics of microbial nitrification. It was demonstrated that the reaction fits the Michaelis-Menten model for temperatures from 10 to 35°C, having a temperature optimum at 15°C (K₃ 0.72 mg 1⁻¹ NH₃). Nitrification is unaffected by high dissolved oxygen concentration 38 mg 1⁻¹ O₂ at 30°C) after acclimatisation. Nitrite concentrations > 20 mg 1⁻¹ are inhibitory to the reaction.     

Effect of long term anaerobic and intermittent anaerobic/aerobic starvation on aerobic granules

The effect of long term anaerobic and intermittent anaerobic/aerobic starvation on the structure and activity of aerobic granules was studied. Aerobic granular sludge treating abattoir wastewater and achieving high levels of nutrient removal was subjected to 4-5 week starvation under anaerobic and intermittent anaerobic/aerobic conditions. Microscopic pictures of granules at the beginning of the starvation period presented a round and compact surface morphology with a much defined external perimeter. Under both starvation conditions, the morphology changed at the end of starvation with the external border of the granules surrounded by floppy materials. The loss of granular compactness was faster and more pronounced under anaerobic/aerobic starvation conditions. The release of Ca²⁺ at the onset of anaerobic/aerobic starvation suggests a degradation of extracellular polymeric substances. The activity of ammonia oxidizing bacteria was reduced by 20 and 36% during anaerobic and intermittent anaerobic/aerobic starvation, respectively. When fresh wastewater was reintroduced, the granules recovered their initial morphology within 1 week of normal operation and the nutrient removal activity recovered fully in 3 weeks. The results show that both anaerobic and intermittent anaerobic/aerobic conditions are suitable for maintaining granule structure and activity during starvation.     

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.     

Basic studies on carbon dioxide/air control system for activated sludge processes

Basic facts about the production of carbon dioxide by activated sludge in an aeration tank were gathered with a continuous-flow-type apparatus, and the dynamic characteristics of activated sludge was analyzed with a batch-type aeration tank. In addition, a carbon dioxide/air control system was designed and applied to an activated sludge pilot plant, where the carbon dioxide in exhausted air was continuously measured with an infrared analyzer. The control system maintained the carbon dioxide concentration in exhausted air within ± 0.005 vol%, with a typical diurnal flow rate pattern in sewage, and maintained the oxygen content in mixed liquor within ± 0.4 mg l⁻¹.     

Control of activated sludge filamentous bulking—III. Effect of sludge loading

Previous experiments have indicated a possibility of controlling the filamentous bulking of activated sludge by cultivating a mixed culture in a system with the plug flow pattern or in a system with a selector. These experiments, however, were conducted at approximately constant sludge load [0·3–0·4 kg BOD₅(kg MLVSS day)⁻¹] and sludge age (4·5–5·0 days) so that the results obtained had only a limited validity. Further experiments were, therefore, carried out in order to find if the previous findings hold also for activated sludge systems with high sludge loads and low sludge ages.The experiments were conducted on four activated sludge systems with different flow patterns: two of them as complete-mixing reactors and the other two as reactors with the plug flow pattern, having dispersion numbers of 0·08 and 0·07. A mixture of starch and peptone was used as the substrate. All systems were operated at sludge ages 2 and 3 days and at sludge loads from 0·5 to 2·3 kg BOD₅(kg MLVSS day)⁻¹.It was found that at sludge loads above 0·5 kg kg⁻¹ day⁻¹ mixed cultures developed in the systems with the plug flow pattern possessed high values of the SVI and contained high portions of filamentous microorganisms. This was found in spite of sufficiently high concentration gradients of the substrate along these systems. It means that plug flow and, consequently, the concentration gradient of the substrate along the activated sludge system may not be the only factor in controlling the selection of microorganisms in a mixed culture. An additional factor must exert an influence on the population dynamics and selection of microorganisms in mixed cultures at high sludge loads and low sludge ages.The experiments have confirmed findings of others that values of the SVI depend on the sludge load. However, different trends of dependence were observed in the systems with different flow patterns. While in the complete-mixing systems the SVI decreased with increasing sludge loading in the systems with the plug flow pattern it increased.A greater phosphorus demand was observed in plug flow systems than in those with complete mixing.     

The effect of pretreatments on the virus contents of sewage samples

By isolations of virus from sewage and sludge samples, which had been fractionated by centrifugation at 9000 rev min^-1, it was found that virus may be particle bound and recoverable in the precipitates. An exception was the secondary sludge of the activated sludge treatment. It was thus indicated that removal of a part of a sewage or sludge sample before examination may make true quantitative estimations impossible.Re-examination of samples after storage at −20°C for 6 months gave remarkably poor virus recoveries.     

Benthal stabilization of waste activated sludge

Laboratory studies were performed on the stabilization of waste activated sludge solids in a deposit formed at the bottom of an aerated water column. Conclusions reached were as follows: (1) As long as dissolved oxygen is maintained in the water column, the benthal process is stable and odor free. (2) At temperatures within the range of 20 ± 1°C, an average benthal stabilization rate of 79.4 g m⁻² d⁻¹ (biodegradable solids) can be attained if solids are not limiting. (3) Under conditions stated in conclusion (2), as much as 63% of total carbon stabilization can occur via methane formation. (4) A year-old benthal deposit of waste activated sludge solids can be expected to have a solids percentage of about 2.3%     

Characterization of alginate-like exopolysaccharides isolated from aerobic granular sludge in pilot-plant

To understand functional gel-forming exopolysaccharides in aerobic granular sludge, alginate-like exopolysaccharides were specifically extracted from aerobic granular sludge cultivated in a pilot plant treating municipal sewage. The exopolysaccharides were identified by the FAO/WHO alginate identification tests, characterized by biochemical assays, gelation with Ca²⁺, blocks fractionation, spectroscopic analysis as UV-visible, FT-IR and MALDI-TOF MS, and electrophoresis. The yield of extractable alginate-like exopolysaccharides was reached 160 ± 4 mg/g (VSS ratio). They resembled seaweed alginate in UV-visible and MALDI-TOF MS spectra, and distinguished from it in the reactions with acid ferric sulfate, phenol-sulfuric acid and Coomassie brilliant blue G250. Characterized by their high percentage of poly guluronic acid blocks (69.07 ± 8.95%), the isolated exopolysaccharides were capable to form rigid, non-deformable gels in CaCl₂. They were one of the dominant exopolysaccharides in aerobic granular sludge. We suggest that polymers play a significant role in providing aerobic granular sludge a highly hydrophobic, compact, strong and elastic structure.     

The anaerobic degradation of detergent range fatty alcohol ethoxylates. Studies with C-labelled model surfactants

The anaerobic degradation of fatty alcohol polyglycol ethers was studied in a model sludge digester employing stearyl alcohol ethoxylate which was ¹⁴C-labelled either in the alkyl or in the heptaglycol chain. After 4-weeks' incubation of the ¹⁴C-compounds at 35°C in the presence of raw sludge as additional digestible substrate more than 80% of the initial radioactivity was found as methane and carbon dioxide. In addition, the major part of radioactivity in the digested sludge, corresponding to nearly 10% of added ¹⁴C, was attributable to biomass so that ultimate degradation of the two model surfactants amounted to more than 90%. Analysis of the small fraction of radiolabelled metabolites in the sludge supernatant allowed conclusions with regard to the anaerobic degradation route of linear alcohol ethoxylates. After primary biodegradation of the surfactant molecule by scission into the alkyl and poly(ethylene glycol) moieties the further biodegradation of the latter seems to proceed as under aerobic conditions, i.e. via oxidative or hydrolytic depolymerization steps. Eventually, ultimate biodegradation of the obtained monomers (C₂-units) leads to the formation of the gaseous end products.     

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.     

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.     

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 biodegradability of para-dichlorobenzene and its behaviour in model activated sludge plants

Confirmation of the ready and ultimate biodegradation of para-dichlorobenzine (pDCB) was obtained from the Organisation for Economic Cooperation and Development (OECD) closed bottle test. The fate of pDCB during primary sedimentation of raw sewage was investigated in a simple laboratory study, and the results indicated that the major proportion of the material (> 67%) remained in the supernant liquor, and would therefore undergo biological treatment. The removal of pDCB in the OECD confirmatory test was found to be consistently high (97.1 ± 2.3%). Mass balance results indicated that, at a high aeration rate (21 min⁻¹), the major mechanism for removal of pDCB was volatilization (66.6 ± 19.3%). The fate of pDCB in a modified porous pot test, operated at a lower aeration rate, and at temperatures of 8, 15, and 20°C and sludge retention times (SRT) of 3 and 6 days, was also investigated. Once again the overall removal was found to be consistently >95%. Mass balance results suggested that, under normal operating conditions, the major proportion of pDCB (>76%) had been removed by biodegradation. The amount of pDCB removed by volatilization showed a slight dependence on aeration rate, but only became significant (63 ± 4%) when adverse operating conditions prevailed (i.e. 8°C, at a 3 day SRT). Radioisotopic studies confirmed the presence of pDCB degrading bacteria in activated sludge taken from a porous pot continuously dosed with pDCB. The results of a similar test using sludge from an undosed porous pot suggested that acclimatization to pDCB was rapid, occurring within 2 days. No adverse effects on sewage treatment processes, due to the presence of pDCB, were observed.     

Some practical aspects of the “biocarbon process”

In the presence of granular activated carbon the aerobic decomposition of organic compounds is facilitated (Koppe et al., 1974a,b). As could be shown by many laboratory tests several effects are given. First of all the faster adaptation of the bacteria has to be mentioned which is most important with substances which are difficult to be degraded like pentaerythrite. Additionally the degradation efficiency of the activated sludge process will be increased and equalized by granular activated carbon. Finally the effect of low temperatures can be compensated by the presence of granular activated carbon. With domestic wastewater for instance full nitrification could be obtained at a temperature of 4°C.Adsorption of organic compounds cannot be the explanation because the effect could be observed during several months whereas the calculated adsorption capacity had to be exhausted after two days. True causes are that specialized bacteria are retained in the pores and niches of the activated carbon in the lag-phase, that micro-organisms find area to grow in the log-phase (approximately 100 m²/m³ with 1 kg m⁻³ granular activated carbon) and that the enrichment of some exoenzymes at the activated carbon occurs (compare Figs. 1 and 2; and Müller & Sekoulov, 1975; Lue-Hing et al., 1976; Larsson et al., 1976; Rincke & Wolters, 1970). Since micro-organisms and activated carbon intimately react, this modification of the activated sludge process was called “biocarbon process” which has to be distinguished from the process with powdered activated carbon (de Walle & Chian, 1977).After completion of the laboratory tests, full-scale tests were started. In full-scale tests several difficulties had to be overcome. In some cases it was difficult to avoid the sedimentation of the granular activated carbon in the aeration tanks. Another task was the separation of the carbon from the surplus activated sludge. Finally it was of interest to determine how much carbon is lost by wet abrasion to be able to make a feasibility calculation. The full-scale tests have been performed in two wastewater treatment plants of the Ruhrverband: Nordenau and Günne.     

The effect of sludge digestion on virus infectivity

The rate of inactivation of coxsackievirus B3 during anaerobic sludge digestion was found to be 2 log units in 24 h at pH 7.0 and 35°C. The rate indicates that virus inactivating factor(s) were produced in the digestion. The virus inactivation was enhanced if the digester sludge was pasteurized at 60°C for 1 h. The reason why digesters at sewage treatment plants do not completely inactivate virus is discussed in terms of the continuous nature of digester operation.     

The effect of temperature on the removal of non-ionic surfactants during small-scale activated-sludge sewage treatment—I. Comparison of alcohol ethoxylates with a branched-chain alkyl phenol ethoxylate

The degree of biodegradability of three non-ionic surfactants has been assessed using small-scale activated-sludge sewage treatment plants. The effect of temperature on biodegradability was studied by operation at 15°, 11° and 8°C.The two alcohol ethoxylate surfactants tested were well degraded at all three operating temperatures. The alkyl phenol ethoxylate surfactant tested was well degraded at 15°C but at lower temperatures the biodegradability was dependent on concentration. At 5 mg 1⁻¹, greater than 90 per cent removal was achieved but at 20 mg 1⁻¹ the degree of removal fluctuated between 40 and 95 per cent at 11°C and between 20 and 80 per cent at 8°C.The present results have been compared with those obtained using the same surfactants in community trials at a small sewage works employing biological filtration. Although a similar temperature effect was observed with the alkyl phenol ethoxylate, the biodegradabilities obtained in the laboratory were consistently greater than those obtained at the sewage works, perhaps because of the constant conditions of the laboratory test.     

An assessment of the fate, behaviour and environmental risk associated with sunscreen TiO₂ nanoparticles in UK field scenarios

The fate of Ti was examined in an activated sludge plant serving over 200,000 people. These studies revealed a decrease of 30 to 3.2 μg/L of Ti < 0.45 μm from influent to effluent and a calculated Ti presence of 305 mg/kg DW in wasted sludge. Thus, using sludge as a fertiliser would result in a predicted deposition of up to 250 mg/m² of Ti to soil surfaces using a recommended maximal agricultural application rate. Given the major use of TiO₂ in many industrial and domestic applications where loss to the sewer is possible, this measured Ti was presumed to have been largely TiO₂, a proportion of which will be nanoparticle sized. To assess the behaviour of engineered nanoparticle (ENP) TiO₂ in sewage and toxicology studies, Optisol (Oxonica Materials Ltd) and P25 (Evonik Industries AG), which are representative of forms used in sunscreen and cosmetic products, were used. These revealed a close association of TiO₂ ENPs with activated sludge. Using commercial information on consumption, and removal rates for sewage treatment, predictions were made for river water concentrations for sunscreen TiO₂ ENPs for the Anglian and Thames regions in Southern England. The highest predicted value from these exercises was 8.8 μg/L for the Thames region in which it was assumed that one in four people used the recommended application of sunscreen during a low flow (Q95) period. Ecotoxicological studies using potentially vulnerable species indicated that 1000 μg/L TiO₂ ENP did not affect the viability of a mixed community of river bacteria in the presence of UV light. Direct exposure to TiO₂ ENPs did not impair the immuno-effectiveness of earthworm coelomocyte cells at concentrations greatly above those predicted for sewage sludge.     

Characterization of sewage plant hydrocolloids using asymmetrical flow field-flow fractionation and ICP-mass spectrometry

Asymmetrical flow field-flow fractionation (AF⁴) was applied to characterize aquatic colloids from biological sewage plants and to infer information of colloidal loads, sources, and sinks within the plants, resp. the colloidal interaction with the aqueous phase and the sewage sludge. To characterize the colloids further, especially the distributions of colloid associated heavy metals, the AF⁴ system was coupled to an inductively coupled plasma mass spectrometer (ICP-MS). The size distribution is determined by AF⁴ with UV absorbance and fluorescence detection after a calibration by monodisperse polystyrene sulfonate standards (PSS). Samples from different sewage plants and from different depths and locations within a plant were compared. The fulvic/humic acid fraction with a particle diameter d_p<10 nm appeared to be comparable in all samples and decreases only slightly along the plants, whereas larger colloids with d_p>10 nm almost completely passed into the sewage sludge. The concentrations of the initial colloidal heavy metals decreased along the plants.