Effect of filter media thickness on the performance of sand drying beds used for faecal sludge management.

The effect of sand filter media thickness on the performance of faecal sludge (FS) drying beds was determined in terms of: dewatering time, contaminant load removal efficiency, solids generation rate, nutrient content and helminth eggs viability in the dried sludge. A mixture of ventilated improved pit latrine sludge and septage in the ratio 1:2 was dewatered using three pilot-scale sludge drying beds with sand media thicknesses of 150, 250 and 350 mm. Five dewatering cycles were conducted and monitored for each drying bed. Although the 150 mm filter had the shortest average dewatering time of 3.65 days followed by 250 mm and 350 mm filters with 3.83 and 4.02 days, respectively, there was no significant difference (p > 0.05) attributable to filter media thickness configurations. However, there was a significant difference for the percolate contaminant loads in the removal and recovery efficiency of suspended solids, total solids, total volatile solids, nitrogen species, total phosphorus, chemical oxygen demand, dissolved chemical oxygen demand and biochemical oxygen demand, with the highest removal efficiency for each parameter achieved by the 350 mm filter. There were also significant differences in the nutrient content (NPK) and helminth eggs viability of the solids generated by the tested filters. Filtering media configurations similar to 350 mm have the greatest potential for optimising nutrient recovery from FS.

Application of microbial indices to assess the performance of a sequencing batch reactor and membrane bioreactor treating municipal wastewater.

Microbial indexes of filamentous bacterial density were evaluated for their potential to act as indicators of sludge settling characteristics for sequencing batch reactor (SBR) and membrane bioreactor (MBR) treatment options. These options were operated using settled sewage over a range of aerated loading rates from 0.05 to 0.4/d and the evolution of protozoan and metazoan populations analysed. A filament density score ranging between 0 and 5, which has previously been applied to conventional activated sludge processes, was shown also to be a useful tool for the SBR and MBR, and was correlated to the settleability of the mixed liquor from both reactors. Due to the hydrodynamics of both systems and the subsequent differences in mixed liquors, optimum performance for each bioreactor was obtained under different operating conditions. Although there was no correlation between the numbers of any given protozoan species and plant operating conditions, there was a clear dependence between operating conditions and protozoan diversity. The highest diversity was found when operating conditions were optimum for both the SBR and MBR.

Optimising the biogas production from leather fleshing waste by co-digestion with MSW.

Waste from the leather industry, known as limed leather fleshing (LF), has a low C:N (3.2) and an alkaline pH of 11.4. This is a major disadvantage for anaerobic digestion due to ammonia toxicity for methanogenesis. This study describes co-digestion of LF with biodegradable fraction of municipal solids waste optimised over a range of C:N and pH to minimise ammonia and to maximise biogas yield. The optimum conditions were found with a blend that provided C:N of 15 and pH of 6.5 and the cumulative biogas yield increased from 560 mL using LF fraction alone, to 6518 mL with optimum blend. At higher pH of 8.5, unionised ammonia was high (2473 mg L(-1)) coincided with poor biogas yield (47 m Ld(-1)) that confirms ammonia toxicity. By contrast at a pH of 4.5 the ammonia was minimum (510 mg L(-1)), but high VFA (26,803 mg L(-1)) inhibited the methanogens. Biomass activity measured using ATP correlated well with biogas yield as reported previously.

Simple and rapid methods to evaluate methane potential and biomass yield for a range of mixed solid wastes.

This paper describes rapid techniques to evaluate the methane potential and biomass yield of solid wastes. A number of solid wastes were mixed to provide a range of C:N ratios. Empirical formulae were calculated for each waste based on the results of chemical analysis and these formulae were used to estimate the COD equivalent and stoichiometric methane potential (SMP). The actual COD and biochemical methane potential (BMP) were determined experimentally for each waste and for both parameters there was a good agreement between the empirical and experimental values. The potential of adenosine triphosphate (ATP) to act as an indicator of biomass yield (mg VSS mg(-1) COD removed) was determined during the anaerobic digestion process. The biomass yield determined from ATP analysis was in the range 0.01-0.25mg VSS mg(-1) COD removed which corroborated well with previously reported studies. Empirical formula based SMP together with ATP measurement were shown to provide rapid methods to replace or augment the traditional BMP and VSS measurements and are useful for evaluating the bioenergy and biomass potential of solid wastes for anaerobic digestion.

Full-scale trials of recycled glass as tertiary filter medium for wastewater treatment.

Pilot-scale trials at a domestic wastewater treatment works compared the performance of three grades of recycled glass (coarse, medium and fine) when used as tertiary filter media for total suspended solids removal (TSS). Fine glass produced the best effluent quality but blinded rapidly and coarse glass could process three times the flow but with a reduction in final effluent quality. The medium glass offered a compromise with similar flow characteristics to the coarse glass, yet still achieve good solids removal, albeit less than the fine glass. Full-scale studies compared the performance of medium glass with the sand medium that is typically used in this application. There was little difference between them in terms of TSS removal, and they both removed around 75% of TSS from the influent, provided that the solids concentration did not exceed 70mg/l. However, the glass media had superior flow characteristics and was able to treat an additional 8-10% of the influent following the backwash cycle. Over the study period, the influent to the filters had an average TSS concentration of 38mg/l and produced an effluent with an average of 15mg TSS/l. In order to design for an average TSS concentration of 20mg/l, the maximum solids loading on the medium should not exceed 0.25kg solids/m(3)/h. Selecting recycled glass as a tertiary filter medium will give a 10% reduction in the amount of media required, compared to sand. It also carries with it the benefits of using a recycled material, and although these are more difficult to quantify they include: reduced CO(2) emissions and use of a more sustainable product that promotes favourable publicity and positive environmental reporting.

Production and characterization of immobilized-biomass carriers, using polyacrylamide.

Whole microbial cells from an activated sludge seed were immobilised in spherical polyacrylamide beads using a shrink/swell procedure which increased cell viability more than 100-fold over the seed material. The process was optimised with respect to the seed concentration and incubation time. It was shown that the swell procedure was essential to achieve good immobilisation and that biofilm attachment to the beads made a negligible contribution to the immobilised biomass. The physical and biological properties of the immobilised beads were determined and found to settle well and resist mechanical abrasion. In addition, their preparation did not prove toxic to the immobilised biomass. The beads produced have many applications for instance in enhancing the biomass in wastewater treatment processes. In addition the immobilisation process could be applied to a wide range of microbial consortia including pure cultures of microorganisms.

Die-off of enteric bacterial pathogens during mesophilic anaerobic digestion.

Conventionally treated sewage sludge may contain high concentrations of potentially pathogenic microorganisms and additional treatment is required to minimise the risks to health if it is to be recycled to agricultural land. Mesophilic anaerobic digestion (MAD) is the most widely used process in the UK for stabilising sludge prior to agricultural recycling, but little is known about the fate of a number of enteric pathogens as the sludge passes through the treatment processes. The aim of this study was to determine the efficiency of MAD in removing the bacterial enteric pathogens, Salmonella senftenberg, Listeria monocytogenes and Campylobacter jejuni which were added as a spike to the digester feedstock, together with the die-off of indigenous Escherichia coli already present in the sludge. The primary sludge digestion stage of MAD was found to achieve a log removal of 1.66 for E. coli, 2.23 for L. monocytogenes and 2.23 for S. senftenberg. However, the extent of die-off was a function of the numbers of pathogens in the feed and as these increased the log removal also increased. The numbers of C. jejuni were not affected by primary sludge digestion. Additional die-off was provided by secondary sludge digestion with log removals of 1.70 for E. coli, 2.10 for S. senftenberg and 0.36 for C. jejuni.

ASExpert: an integrated knowledge-based system for activated sludge plants.

The activated sludge process is commonly used for secondary wastewater treatment worldwide. This process is capable of achieving high quality effluent. However it has the reputation of being difficult to operate because of its poorly understood biological behaviour, variability of input flows and the need to incorporate qualitative data. To augment this incomplete knowledge with experience, knowledge-based systems were introduced in the 1980s however they didn't receive much popularity. This paper presents the Activated Sludge Expert system (ASExpert), which is a rule-based expert system plus a complete database tool proposed for use in activated sludge plants. The paper focuses on presenting the system's main features and capabilities to revive the interest in knowledge-based systems as a reliable means for monitoring plants. Then it presents the methodology adopted for ASExpert validation along with an assessment of testing results. Finally it concludes that expert systems technology has proved its importance for enhancing performance, especially if in the future it is integrated to a modern control system.

Measuring the effect of a hygiene behaviour intervention by indicators of behaviour and diarrhoeal disease.

A social marketing approach used both qualitative and quantitative methods to develop a hygiene behaviour intervention in rural north-east Thailand. Behaviours were preselected from a previous study and the intervention was designed to promote hand washing, especially before feeding a baby, cooking, eating, and after defaecation or cleaning a baby's bottom, and dish washing immediately after eating. A bacteriological indicator (enumerating faecal streptococci using a finger impression technique) was developed to measure changes in hand washing behaviour and observation (spot checks) of dirty dishes to indicate dish washing practice. There was a significant improvement in both behaviours and a significant reduction in diarrhoeal disease as a result of the intervention. Furthermore, both indicators were retrospectively found to be positively related to diarrhoeal disease incidence. However, receiving and being able to recall the intervention messages was not necessarily sufficient to ensure behaviour change, as some adults found it difficult to change old habits. Villages showing the greatest improvement tended to have a stronger sense of community than others and to have more people actively involved in the intervention.

Seasonal effects on the reported incidence of acute diarrhoeal disease in northeast Thailand.

This paper examines the seasonal variation in the reported incidence of acute diarrhoea for selected areas in the northeast of Thailand. Charts are presented which show rainfall, temperature and reported incidence of acute diarrhoea for the period 1982 to 1987. Incidence of diarrhoea appears to be inversely related to a sharp decrease in temperature around January each year. Although rainfall does not appear to have a direct effect on the relative incidence of acute diarrhoea, there is always a consistent reduction during July or August, after the rains have begun. Seasonal changes in climate may be indirectly related to other factors which have an important bearing on diarrhoeal disease. Rainwater collection is an important water source in this region and the affect this has on water use is discussed in relation to faeco-oral disease transmission.

PIP: Researchers compared data on acute diarrhea incidence with data on rainfall and temperature of 10 provinces and 15 districts of Khon Kaen province in northeast Thailand to determine the link between seasonal weather patterns and reported incidence of acute diarrhea. The relative incidence of diarrhea decreased with age in the winter while it increased with age in the hot season and early in the rainy season. Indeed reported incidence of diarrhea peaked in January for children 2 years old. The researchers suggested that rotavirus was the leading causative agent in this age group since it is common in cooler months and in children 2 years old. Another peak occurred in April-June when temperatures are high and early in the rainy season. This peak involved mainly adults. The researchers believed enterotoxigenic Escherichia coli, Campylobacter jejuni, and Shigella were the leading causative agents. The researchers posed a possible explanation for this April-June peak. In the beginning of the wet season, households changed their water source from groundwater to rainwater and began to use it differently. Further, they often stored water separately from rainjars. It was more likely that this water was more contaminated than stored rainwater or groundwater. In addition, fecal bacteria transmitted by food preparation and utensil handling and the rising humidity fostered its growth. These events may have accounted for the increased incidence of diarrhea in the early rainy season. A steep reduction in diarrhea incidence occurred around the middle of the wet season (July and August). Research has shown that increased water quantity may be better in reducing diarrhea incidence than only improving water quality. Furthermore, it also demonstrated that bacteriological quality of water stored in rainjars was better than water from shallow wells.

The faecal coliform fingertip count: a potential method for evaluating the effectiveness of low cost water supply and sanitation initiatives.

A rapid and simple procedure for the enumeration of faecal coliform bacteria present on the fingertips is described. The distribution of individual faecal coliform fingertip counts within households has been compared with a number of variables related to the availability of water supply and sanitation facilities. This showed a strong association between the fingertip count and the possession of in-house water connections. It is proposed that this test might provide a reliable indication of the effectiveness of certain public health interventions.

Effect of starvation on transport, membrane potential and survival of Staphylococcus epidermidis under anaerobic conditions.

When washed suspensions of Staphylococcus epidermidis were starved under anaerobic conditions the viability declined to less than 10% within 12 h. Although RNA was slightly degraded during this period the principal substrate for endogenous metabolism was protein and the intracellular amino acid pool. The adenylate energy charge and the ability to transport serine declined markedly within the first 6 h of starvation. With the majority of batches of organism investigated the membrane potential, as measured by the accumulation of Cs+ by valinomycin-treated organisms, also decreased significantly during this period. Addition of glucose or serine during starvation reversed these effects to varying extents provided that feeding took place during an early phase (2 h) of starvation. There was no apparent correlation between the magnitude of the membrane potential and viability.

Anaerobic transport of serine and 2-aminoisobutyric acid by Staphylococcus epidermidis.

A membrane-bound ATPase detected in extracts of anaerobically grown Staphylococcus epidermidis was inhibited by a variety of compounds which inhibit ATPases in other organisms. Serine and 2-aminoisobutyric acid (AIB) were shown to enter the organism via the same transport system. The transport of AIB, the membrane potential and the transmembrane pH gradient were partially or completely abolished by the same inhibitors and also by uncoupling agents and lipid-soluble ions. It is proposed therefore that this ATPase generates and maintains an electrochemical gradient of protons across the cytoplasmic membrane of S. epidermidis capable of driving AIB uptake. Studies of AIB-induced proton movements suggested that AIB enters via a proton symport mechanism.