Inoculum standardization for biodegradability tests

Summary To obtain standardized inoculum with constant characteristics for biodegradability test, Actizym powder was used instead of sludge. A predigestion and stabilization step were introduced to prepare “activated Actizym”, before inoculation and addition of test chemical. The inoculum obtained is similar to activated sludge in its microbiological and physical aspects. The reproducibility of the method was prooved and several controls were made using chemicals with different biodegradation rates.     

Biodegradation of o-Benzyl-p-Chlorophenol

The extent of biodegradation of o-benzyl-p-chlorophenol, marketed as a germicide under the name Santophen((R)) 1 (Monsanto Co.), in river water, sewage, and activated sludge was determined. Biodegradation was assessed by use of a colorimetric procedure for phenolic materials, carbon analysis, and CO(2) evolution. In unacclimated river water, 0.1 mg of Santophen 1 per liter was degraded within 6 days. In sewage, 0.5 and 1.0 mg/liter levels of Santophen 1 were degraded in 1 day. Acclimated activated sludge achieved 80% biodegradation of 1.0 mg/liter Santophen 1 in 8 h and 100% in 24 h. When effluent from a semicontinuous activated sludge unit, acclimated to 20 mg of Santophen 1 per liter was used as the inoculum for the CO(2) evolution procedure, 60% of the total theoretical CO(2) was evolved from Santophen 1. Based on the results of these studies, indicating Santophen 1 to be readily biodegraded in at least four biological systems, the continued use of present levels of Santophen 1 should present no significant environmental problems.     

Performance evaluation of mixed and pure microbial inocula as surrogate culture in a BOD5 test

The use of activated sludge (AS) to assess the environmental impact of chemicals and wastewaters suffers from several drawbacks related to the heterogeneity, absence of standardization and health risk associated with such a mixed sewage population. To search for reliable testing inoculum alternatives, the potential of a standardized soil inoculum and a pure culture (Pseudomonas sp.), as reference material in a 5-day BOD test, was evaluated using the dilution and manometric methods and testing the standard glucose-glutamic acid solution. The results obtained for the BOD₅ of the standard solution fall in the range referred for the AS in the standards of the cited methods, demonstrating the potential of these inocula as an alternative to AS in the BOD₅ test. Moreover, testing on real wastewater samples showed highly significant correlations (P < 0.001) between the BOD₅ values obtained with these two inocula and with the standard inoculum. Analysis of metabolic patterns also pointed to a similar catabolic profile between soil inoculum and AS and to the potential of a mixed Pseudomonas-soil inoculum as an AS surrogate culture.     

Strategy for minimization of excess sludge production from the activated sludge process

Increased attention has been given to minimization of sludge production from activated sludge process since environmental regulations are being more and more stringent in relation to excess sludge disposal. In a biological process, the more organic carbon utilized in carbon dioxide production, the fewer sludge produced, and vice versa. This paper, therefore, reviews strategies developed for minimization of excess sludge production, such as oxic-settling-anaerobic process, high dissolved oxygen process, uncoupler-containing activated sludge process, ozonation-combined activated sludge process, control of sludge retention time and biodegradation of sludge in membrane-assisted reactor. In these modified activated sludge processes, excess sludge production can be reduced by 20-100% without significant effect on process efficiency and stability. It is expected that this paper would be helpful for researchers and engineers to develop novel and efficient operation strategy to minimize sludge production from biological systems.     

A rationale for the appropriate amount of inoculum in ready biodegradability tests

Several screening methods at the so-called ready biodegradability level are suitable to test poorly soluble substances. Typical for these tests is that mineralization is evaluated from monitoring oxygen uptake or carbon dioxide production. Unfortunately, they suffer from a rather low precision in the calculated percentage of mineralization caused by subtracting a too high inoculum control measurement from the response in the test system. Criteria for blank oxygen consumption, due to the metabolic activity of the inoculum, are proposed from which maximum amounts of activated sludge or secondary effluent per litre test medium can be derived to be used as an appropriate inoculum. Both for current and future standardized tests the precision of the method can be kept within acceptable margins. Inoculum material was sampled from 40 communal biological waste water treatment plants. From endogenous respiration rates it was derived that the concentration of secondary effluent in the Closed Bottle Test can be increased up to 50 mL/L but that in respirometry tests inoculated with activated sludge the appropriate concentration is 10 mg/L dry matter or below, depending of the design of the test system.List of abbreviations BOD biological oxygen demand - CBT Closed Bottle Test - C _as inoculum concentration in mg dry solids of activated sludge per litre test medium - C _ef inoculum concentration in ml secondary effluent per litre test medium - C _ss dry weight content of activated sludge (g/L) - CFU colony forming units - DO_7d dissolved oxygen concentration (mg/L) after 7 days - ISO International Organization for Standardization - NEN Dutch Organization for Standardization - O _c oxygen capacity in mg oxygen per litre vessel volume - OECD Organisation for Economic Co-operation and Development - Ox _as oxygen consumption after one week in mg oxygen per mg dry weight activated sludge - Ox _ef oxygen consumption after one week in mg oxygen per mL secondary effluent - Ox _ef [n] oxygen consumption after one week in mg oxygen per n mL secondary effluent - Ox _flask oxygen uptake in mg per litre flask volume - RBT Ready Biodegradability Test - SLR sludge loading rate in kg O₂/kg dry weight·d - ThOD theoretical oxygen demand - TPCBT Two Phase Closed Bottle Test - V _a volumes of air and water per litre vessel - V _w volume, respectively - _a concentration of oxygen in air at 20° C and 101.5 kPa - _s saturation oxygen concentration in te aqueous phase     

Aerobic and anaerobic biodegradability of 1-Anthraquinone sulphonate

 The present work investigates 1-anthraquinone sulphonate (1-AS) biodegradation under (i) aerobic conditions using domestic activated sludge as inoculum, (ii) anaerobic conditions using sludge from an anaerobic domestic wastewater treatment digestor in a sulphate-containing or methanogenic environment, (iii) a combination of anaerobic followed by aerobic conditions. The process was evaluated in terms of primary degradation, i.e. 1-AS elimination and ultimate degradation, as total dissolved organic carbon removal. It was shown that aerobic conditions lead to the complete primary and ultimate degradation, of 1-AS. By contrast, neither under sulphato-reductive nor methanogenic conditions does anaerobic digestion lead to the significant degradation of 1-AS. The use of anaerobic treatment followed by aerobic treatment did not improve degradation. Indeed aerobic post-treatment resulted in the re-appearance of pollutant in the medium even though this had been partly degraded under anaerobic conditions. Received: 12 October 1995/Received revision: 18 December 1995/Accepted: 8 January 1996     

Photo-Fenton process for excess sludge disintegration

For the minimization of excess sludge in activated sludge process, the photo-Fenton reaction was applied as a novel technique and its effectiveness was experimentally examined. A batch study was conducted to elucidate parameters governing the activated sludge integration by the photo-Fenton process such as the concentrations of sludge, Fe ion and hydrogen peroxide. It was found that the chemical oxidation sludge disintegration by photo-Fenton reaction could be divided into two phases. At the beginning of the photo-Fenton process the dissolved chemical oxygen demand (COD) increased. This is due to the discharge of organic compounds occurred by oxidative decomposition of the cell wall of a microbe in the sludge. The COD reached the maximum and then decreased. In this phase, mineralization of dissolved organic substances by the photo-Fenton reaction might be dominant. The occurrence of mineralization was suppressed as the initial MLSS increased. The Fe dosage enhanced the sludge disintegration by the photo-Fenton reaction as well as the H₂O₂ dosage. The soluble total organic carbon (TOC) of the solution increased monotonously. The results suggest that Fe ions might be entrapped into the activated sludge. It may be concluded that photo-Fenton reaction is one of feasible processes for disintegration of excess activated sludge.     

Survey of the Anaerobic Biodegradation Potential of Organic Chemicals in Digesting Sludge

The degradation potential of 77 organic chemicals under methanogenic conditions was examined with an anaerobic digesting sludge from the United Kingdom. Degradation was assessed in terms of net total gas (CH₄ plus CO₂) produced, expressed as a percentage of the theoretical production (ThGP). The compounds tested were selected from various chemical groups and included substituted phenols and benzoates, pesticides, phthalic acid esters, homocyclic and heterocyclic ring compounds, glycols, and monosubstituted benzenes. The results obtained were in good agreement with published surveys of biodegradability in U.S. digesting sludges and other methanogenic environments. In general, the presence of chloro or nitro groups inhibited anaerobic gas production, while carboxyl and hydroxyl groups facilitated biodegradation. The relationship between substituent position and susceptibility to methanogenic degradation was compound dependent. The following chemicals were completely degraded (≥80% ThGP) at a concentration of 50 mg of carbon per liter: phenol, 2-aminophenol, 4-cresol, catechol, sodium benzoate, 4-aminobenzoic acid, 3-chlorobenzoic acid, phthalic acid, ethylene glycol, diethylene glycol, triethylene glycol, sodium stearate, and quinoline. 3-Cresol, 4-chlorobenzoic acid, dimethyl phthalate, and pyridine were partially degraded. Although the remaining chemicals tested were either persistent or toxic, their behavior may differ at more environmentally realistic chemical-to-biomass ratios. Our findings suggest that biodegradability assessments made with sludge from one source can be extrapolated to sludge from another source with a reasonable degree of confidence and should help in predicting the fate of an organic chemical during the anaerobic digestion of sewage sludge.     

Effect of type of granular activated carbon on DOC biodegradation in biological activated carbon filters

The purpose of this paper is to clarify the effect of the two different GAC types (steam activated or chemically activated) on DOC biodegradation in biological activated carbon (BAC) columns. For this purpose, raw water taken from a surface reservoir was fed to continuous-flow lab-scale biofiltration columns which were run for more than 18,000 bed volumes. The effect of pre-ozonation on DOC removal was also evaluated. Experimental results showed that biological activity inside the BAC columns extended the service life and the choice of filter material was crucial in BAC systems. The DOC biodegradation was higher in thermally activated carbon columns compared to the chemically activated one. The ability of GAC to better adsorb and retain organic compounds increased the chance of biodegradation. Contrary to expectations, pre-ozonation did not significantly enhance DOC biodegradation. Despite the high increase in biodegradable dissolved organic carbon (BDOC) upon ozonation, overall DOC biodegradation efficiencies did not differ from raw water. Overall, the DOC biodegradation in columns was higher than in most of the studies. This observation was primarily attributed to the low specific ultraviolet absorption (SUVA) values in raw water indicating a high biodegradability.     

Development of ecotoxicity assay based on inhibition of respiring activity in microbial community using XTT reduction

The aim of this study is to develop ecotoxicity assay for evaluating the influence of chemicals on a microbial ecosystem based on XTT reduction inhibition (XTT assay). XTT reduction method is used for quantification of the microbial respiratory activity. Since the XTT assay indicates the inhibition of microbial respiratory activity, it could evaluate the toxicity of chemicals. Suitable conditions for the XTT assay were determined to be 200 mg/L of particulate organic carbon as test microbe concentration and 15 min of assay time using activated sludge. Toxicities of several chemicals evaluated by activated sludge as test microbes were examined under these conditions. Sensitivity for the toxicity evaluated by the XTT assay using activated sludge microbes was almost the same value was that for the OECD activated sludge respiration inhibition test (ASRI test). XTT assay was also applied for evaluating the influence of chemicals on the soil microbial community and the XTT assay was used to evaluate a median effective concentration (EC(50)) value of 3,5-dichlorophenol (3,5-DCP). The EC(50) value of 3,5-DCP was almost the same as the value using activated sludge as test microbes. These results suggest that the XTT assay using both mixed cultures of non-contaminated environments and chemical extracts from various contaminated environments could evaluate the influence on microbial ecosystems affected by toxic chemicals.     

Development and Evaluation of an Online CO2 Evolution Test and a Multicomponent Biodegradation Test System

Well-established biodegradation tests use biogenously evolved carbon dioxide (CO₂) as an analytical parameter to determine the ultimate biodegradability of substances. A newly developed analytical technique based on the continuous online measurement of conductivity showed its suitability over other techniques. It could be demonstrated that the method met all criteria of established biodegradation tests, gave continuous biodegradation curves, and was more reliable than other tests. In parallel experiments, only small variations in the biodegradation pattern occurred. When comparing the new online CO₂ method with existing CO₂ evolution tests, growth rates and lag periods were similar and only the final degree of biodegradation of aniline was slightly lower. A further test development was the unification and parallel measurement of all three important summary parameters for biodegradation—i.e., CO₂ evolution, determination of the biochemical oxygen demand (BOD), and removal of dissolved organic carbon (DOC)—in a multicomponent biodegradation test system (MCBTS). The practicability of this test method was demonstrated with aniline. This test system had advantages for poorly water-soluble and highly volatile compounds and allowed the determination of the carbon fraction integrated into biomass (heterotrophic yield). The integrated online measurements of CO₂ and BOD systems produced continuous degradation curves, which better met the stringent criteria of ready biodegradability (60% biodegradation in a 10-day window). Furthermore the data could be used to calculate maximal growth rates for the modeling of biodegradation processes.     

An analysis of the bacterial community in a membrane bioreactor fed with photo-Fenton pre-treated toxic water

A photo-Fenton-membrane bioreactor (MBR) coupled system is an innovative tool for the treatment of wastewater containing high quantities of contaminants. In this paper, wastewater with 200 mg l^?1 of dissolved organic carbon (DOC) of a selected mixture of five commercial pesticides: Vydate^®, Metomur^®, Couraze^®, Ditimur-40^®, and Scala^® was treated by combining photo-Fenton and MBR. The effect of photo-treated pollutants on MBR operation was investigated by studying the population changes that occurred with time in the activated sludge of the biological system. Pre-treatment with photo-Fenton was carried out (only up to 34% of mineralization of DOC) and, after MBR treatment, 98% of biodegradation efficiency was obtained. During the biological treatment, little changes in the activated sludge population were detected by DGGE analysis, maintaining acceptable biodegradation efficiency, which points out the robustness of the MBR treatment versus changes in feed composition.     

Biodegradation of coal slurry transport wastewater organics

Summary Activated sludge was successful in reducing the levels of dissolved organic carbon (DOC) in coal slurry wastewaters. DOC removal by the activated sludge ranged from 61% to 97% with a large percentage (21–41%) of this DOC being completely metabolized to CO₂. Second order kinetic constants (k ₂) developed for DOC removal ranged from 1.39·10^–4 to 2.30·10^–1 liter·day^–1·(mg of sludge)^–1, providing evidence that biological treatment was an effective mechanism for reducing the pollution potential of the slurry wastewaters. After treatment with activated sludge a residual DOC remained in the wastewater and data from ultrafiltration studies indicated that this residual carbon was of MW>1000. The activated sludge preferentially removed the lower (MW<1000) molecular weight compounds and the higher molecular weight DOC was more resistant to biological attack. However, extended acclimation (greater than 1 month) enabled the activated sludge to remove the higher molecular weight DOC from the slurry wastewaters.     

Inoculum selection influences the biochemical methane potential of agro-industrial substrates

Obtaining a reliable estimation of the methane potential of organic waste streams in anaerobic digestion, for which a biochemical methane potential (BMP) test is often used, is of high importance. Standardization of this BMP test is required to ensure inter-laboratory repeatability and accuracy of the BMP results. Therefore, guidelines were set out; yet, these do not provide sufficient information concerning origin of and the microbial community in the test inoculum. Here, the specific contribution of the methanogenic community on the BMP test results was evaluated. The biomethane potential of four different substrates (molasses, bio-refinery waste, liquid manure and high-rate activated sludge) was determined by means of four different inocula from full-scale anaerobic digestion plants. A significant effect of the selected inoculum on the BMP result was observed for two out of four substrates. This inoculum effect could be attributed to the abundance of methanogens and a potential inhibiting effect in the inoculum itself, demonstrating the importance of inoculum selection for BMP testing. We recommend the application of granular sludge as an inoculum, because of its higher methanogenic abundance and activity, and protection from bulk solutions, compared with other inocula.     

The investigation of effect of organic carbon sources addition in anaerobic–aerobic (low dissolved oxygen) sequencing batch reactor for nutrients removal from wastewaters

The effect of addition of organic carbon sources (acetic acid and waste activated sludge alkaline fermentation liquid) on anaerobic–aerobic (low dissolved oxygen, 0.15–0.45 mg/L) biological municipal wastewater treatment was investigated. The results showed that carbon source addition affected not only the transformations of polyhydroxyalkanoates (PHA), glycogen, nitrogen and phosphorus, but the net removal of nitrogen and phosphorus. The removal efficiencies of TN and TP were, respectively, 61% and 61% without organic carbon source addition, 81% and 95% with acetic acid addition, and 83% and 97% with waste activated sludge alkaline fermentation liquid addition. It seems that the alkaline fermentation liquid of waste biosolids generated in biological wastewater treatment plant can be used to replace acetic acid as an additional carbon source to improve the anaerobic–aerobic (low dissolved oxygen) municipal wastewater nutrients removal although its use was observed to cause a slight increase of effluent BOD and COD concentrations.     

Stoichiometric analysis of dissolved organic carbon flux into storage and growth in aerobic granules culture

This study attempted to address a fundamental question of whether metabolic behaviors of aerobic granules are different from their counterparts, such as activated sludge and biofilms. A series of respirometric experiments were carried out using mature aerobic granules with mean sizes of 0.75–3.4 mm. Results suggested that metabolism of aerobic granules comprised three consecutive phases: (i) conversion of external dissolved organic carbon to a poly-β-hydroxybutyrate-like substance; (ii) growth of aerobic granules on the stored poly-β-hydroxybutyrate-like substance derived from phase I, and (iii) subsequent endogenous metabolism of aerobic granules. The stoichiometric analysis revealed that the conversion yields of external dissolved organic carbon to the poly-β-hydroxybutyrate-like substance, the growth yields of biomass on storage, and the overall growth yields of biomass on external dissolved organic carbon were not significantly correlated to the sizes of aerobic granules, i.e., the metabolism of aerobic granules would be size independent. The conversion coefficients and growth rates of aerobic granules were found to be comparable with those reported in the activated sludge and biofilms cultures, indicating that there would not be significant difference in the metabolisms of aerobic granules over activated sludge and biofilms. This information will be useful for modeling and designing aerobic granular sludge processes.     

TREATABILITY, TOXICITY AND BIODEGRADABILITY TEST METHODS

1. This review confirms that treatability and biodegradability test methods have been cited extensively in the literature. It is clear that the method selected depends on the specific objectives of the test, i.e. the determination of whether a substance is toxic, biodegradable or treatable. Factors that have to be considered when selecting the test methods are the cost of performing the test, the time and resources involved, and the accuracy required. It often appears that more extensive simulation studies are required after initial screening tests have been performed. 2. Many of the enzyme and bacterial growth tests which have been developed for monitoring or screening of toxicants and their persistence in water and wastewaters have been reviewed. Most of these tests are rapid, inexpensive, and reproducible. Most of the biochemical and microcalorimetric approaches, although promising, are still in their infancy as regards toxicity testing. Therefore, biological testing still appears to be most suitable for routine assessment. 3. Micro-organisms are particularly suitable for use in toxicity testing of chemicals as they are inexpensive to culture, have rapid growth rates, and usually provide reproducible results (Vaishnav & Korthals, 1990). Many bioassays have been developed to evaluate the toxicity and treatability of municipal and industrial effluents. Numerous single species tests have been recommended by several authors (Dutka et al., 1983; Beaubien et al. 1986). Such approaches are mainly based on the belief that, by selecting the most sensitive species and by using appropriate factors to allow for variability not included in the test, the highest levels of biological organization will be adequately protected. Single species tests are now quite well established, and when properly used, are easy to analyse and quantify. However, it has been pointed out (Levin, 1984) that the results obtained from single species tests cannot easily be applied to natural field conditions because the test organisms are extensively laboratory acclimated; also the test conditions provide for optimized growth and survival, a situation unlikely to be found in the field. Moreover, a fundamental problem with this approach is that it assumes that the ecosystem is a collection of single species exposed to toxicants under constant conditions (Cairns, 1982). 4. Multi-species toxicity tests, that is the use of mixed cultures or communities of micro-organisms for a testing protocol, are found to be generally much less sensitive than single species tests (Dutka & Kwan, 1984). It has been claimed that the use of multi-species tests give more realistic results than single species tests, and that single species adaptation and replacement is a common phenomenon in natural systems (Cairns, 1982). 5. Chemical persistence is the other major area of testing involving micro-organisms. Many biodegradation procedures have been developed (UK Standing Committee of Analysts, 1981), but surprisingly little interlaboratory validation of test methods has occurred. Those studies which have attempted validation of test methods have generally found good agreement using either non-biodegradable or readily degradable chemicals. Poor agreement has been reported when testing partially degradable or problematic chemicals, especially when non-acclimated inocula were used (Gerike & Fischer, 1979). Most biodegradation test methods have been criticized as the nonspecific analytical techniques employed require the use of relatively high substrate concentrations (Gledhill, 1987). Chemical degradation may be proportional to concentration, may not occur below certain threshold concentrations, may proceed via alternate metabolic pathways, or may occur more rapidly via alternate metabolic pathways. Studies by Alexander (1985) indicate the persistence of chemicals at realistic environmental concentrations may be different from that at higher concentrations. More realistic test systems which simulate actual ecosystems are needed for studies of microbial effects and persistence. Test systems should incorporate realistic chemical concentrations, natural microbial communities, and as much of the physical structure of the environment as practical. They should also be adapted to routine laboratory use. Most biodegradation studies look at the disappearance of parent material or the formation of metabolic products such as CO₂. Because of the relative high variation and lack of reproducibility of results of die-away tests, and as CO₂ may be a poor method of measuring toxicity because of the slow conversion of organic carbon to CO, both of these approaches have serious limitations when examination of typical environmental chemicals is desired. They also become too laborious, expensive or impractical to be useful in routine test procedures. 6. Respiration is a universally applicable parameter for assessing toxicity to aerobic bacteria. Inhibitory effects on respiration are rapid and can be measured with simple, inexpensive equipment. This method offers some advantages over others in that it requires little attention and also simulates quite closely the conditions found at a wastewater treatment plant. Among the disadvantages are the high concentration of test compound required. In the presence of a readily biodegradable substrate, bacteria respire rapidly, but when the substrate is exhausted the respiration rate falls rapidly to an endogenous level in which the cells are using stored substrate and other expendable cell constituents. If substrate is reintroduced after a long period of endogenous respiration, a lag period may be observed before active growth resumes. It has been found that bacteria are less susceptible to inhibitors when respiring in the endogenous phase, and the effects on growth and cell division cannot be ascertained on cells in this state. Thus in order to maximize sensitivity, the bacteria should be provided with a readily oxidizable substrate so that they are exogenously respiring, growing, and dividing when exposed to the potential inhibitor. Generally, a mixture of substrates similar to those on which the culture would grow naturally is suitable, for example synthetic sewage is commonly used as a growth medium for bacteria from sewage treatment plants. 7. Activated sludge is variable in populations of bacteria and varying results are often reported for inhibition of respiration studies. Within-plant variations can occur from day to day as a result of shifts in the bacterial population, probably caused by changes in the strength of components in the influent. Between plant variations can be greater. Sludge from different sources, and/or grown under different conditions, may also vary in response to inhibitors, because of varying degrees of reaction of some inhibitors with non-living sludge components (Kilroy & Gray, 1992b). Therefore, in practice, it has been usual to consider EC₅₀ values from the inhibition of respiration of activated sludge tests in terms of order of magnitude. 8. Normal short-term tests, including inhibition of respiration of activated sludge, are deliberately carried out using unacclimated micro-organisms. This design is aimed to be as conservative as possible for the purpose of predicting effects in the environment, but may be unnecessarily stringent for the purposes of investigating the treatability of a chemical by activated sludge as no account is taken of the possible reduction in toxicity by acclimation of the micro-organisms to the chemical when it is continuously discharged to the wastewater treatment plant. Unlike chemical determinations where all the reagents can be clearly specified, the microbial inoculum or reagent used in biodegradability testing can rarely be fully characterized and is usually only vaguely specified (e.g. as activated sludge, sewage effluent). The BOD method is not labour intensive, but it does require a 5-day test period. While it does not allow a quick assessment of toxicity, it spans a number of generations of bacteria and is therefore more likely to identify effects on growth and cell division than short-term tests lasting for a few hours. If respiration is slow, the test can be extended (to 20 d) to allow for acclimation of the bacteria to the toxic chemical. Recovery of the respiration in the BOD test has been observed in the presence of some initially inhibitory chemicals by many workers (Busch, 1982; Trudgill et al., 1971). 9. Although useful, short-term tests are limiting as experimental conditions differ considerably from those in full-scale plants. The tests are batch processes in which the progress of a biochemical reaction is investigated over a relatively short time period, from 30 min in respiration inhibition studies to 5 d in BOD toxicity testing. The mixing characteristics therefore exhibit completely plug-flow dispersion. In contrast, the dispersion in full-scale continuous-flow plants approaches either completely mixed or dispersed plug-flow character. Substances described as non-toxic and degradable in short-term tests can exhibit other deleterious effects on full-scale operation. In short-term tests the biodegradation of a substance does not require that micro-organisms should be in any particular physical state, whereas the efficient operation of full-scale activated sludge plants requires that the flocculation of biomass occurs in order that separation of solids from the final effluent is achieved. Some substances can cause deflocculation without inhibiting respiratory activity. Other substances are responsible for the proliferation of filamentous micro-organisms resulting in sludge bulking. For these reasons simulation tests should be carried out in order to determine the treatability of chemical substances in activated sludge plants.     

Inhibition of Biogas Production by Alkyl Benzene Sulfonates (LAS) in a Screening Test for Anaerobic Biodegradability

The effect of the inoculum source on the digestion of linear alkylbenzene sulfonates (LAS) under anaerobic conditions has been investigated. The potential for primary and ultimate LAS biodegradation of anaerobic sludge samples obtained from wastewater treatment plants (WWTPs) of different geographical locations was studied applying a batch test system. It was found that only 4–22% of the LAS added to the batch anaerobic digesters was primarily transformed suggesting a poor primary degradation of the LAS molecule in anaerobic discontinuous systems. Regarding ultimate biodegradation, the addition of LAS to the batch anaerobic digesters caused a reduction on the extent of biogas production. Significant differences in the inhibition extent of the biogas production were observed (4–26%) depending on the sludge used as inoculum. Effect of the surfactant on the anaerobic microorganisms was correlated with its concentration in the aqueous phase. Sorption of LAS on anaerobic sludge affects its toxicity by depletion of the available fraction of the surfactant. LAS content on sludge was related to the total amount of calcium and magnesium extractable ions. The presence of divalent cations promote the association of LAS with anaerobic sludge reducing its bioavailability and the extent of its inhibitory effect on the biogas production.     

Kinetic properties of a commercial and a native inoculum for aerobic milk fat degradation

The aerobic fat biodegradation potential and growth characteristics of a commercial and a native inoculum (activated sludge from a dairy wastewater treatment pond), were evaluated. Batch tests were conducted with a medium based on butter oil, as the sole source of carbon, and mineral salts. Residual fat, biomass and CO(2) production were measured. Overall fat removal values were above 78% for both inocula. The growth kinetics of the commercial and native inocula followed Haldane and Monod models respectively. Both inocula showed a similar behaviour when butter oil concentration was under 360 mg/l; at higher values, the difference between the growth rates increased as a consequence of the inhibition exhibited by the commercial inoculum. The selection of an inoculum for bioaugmentation of bioreactors in the wastewater treatment requires a comprehensive knowledge of their degradation ability and tolerance to fluctuating compounds and of the operational conditions that will be utilized.     

Enhancing denitrification using a carbon supplement generated from the wet oxidation of waste activated sludge

This study compared the effect of four pure carbon supplements on biological denitrification to a liquor derived as a by-product from the wet oxidation (WO) of waste activated sludge. Sequencing batch reactors were used to acclimate sludge biomass, which was used in batch assays. Acetate, WO liquor and ethanol-supplementation generated the fastest denitrification rates. Acetate and WO liquor were efficiently utilised by all acclimated biomass types, while poor rates were achieved with methanol and formate. When comparing an inoculum from an ethanol-supplemented and non-supplemented wastewater treatment plant (WWTP), the ethanol-acclimated sludge obtained superior denitrification rates when supplemented with ethanol. Similarly high nitrate removal rates were achieved with both sludge types with acetate and WO liquor supplementation, indicating that WO liquors could achieve excellent rates of nitrate removal. The performance of the WO liquor was attributed to the variety of organic carbon substrates (particularly acetic acid) present within the liquor.