Odours from wastewater treatment plants comprise a mixture of various gases, of which hydrogen sulphide (H2S) is the main constituent. Microorganisms commonly found in wastewater can degrade sulphurous compounds. Therefore, the use of activated sludge (AS) for odour control offers an alternative to traditional waste gas treatment processes, such as biofilters, bioscrubbers and biotrickling filters, both in practical terms (use of existing facilities) and economically (minimal capital cost). The performance of AS diffusion as a bioscrubber for removing H2S at concentrations at 25, 75 and 150 ppmv was evaluated. Pilot-scale trials were undertaken using parallel 60-L aeration tanks and 20-L clarifier reactors at the Bedford Sewage Treatment Works, Carington, UK. Olfactometry measurements were also carried out to determine whether there was any increase in odour concentration owing to H2S diffusion. Hydrogen sulphide removal rates of 100% were obtained, with no noticeable increase in odour concentration throughout the trials as measured by olfactometry. Odour concentration was highest at the beginning of the trials and lowest during the high H2S dosing period, with similar values being obtained for test and control. It was concluded that AS diffusion is an effective bioscrubber for the removal of H2S odour.

The effects of hydrogen sulfide (H 2 S) diffusion into activated sludge (AS) on odor and volatile organic compound (VOC) concentrations in offgas were studied over an 8-week period. Most VOCs detected in the offgas of both aeration tanks were aromatic hydrocarbons. The VOC concentrations generally decreased when H 2 S was introduced to the AS compared with the control, indicating a negative effect of H 2 S on VOC removal. Two volatile organic sulfur compounds present in the test AS offgas showed an increase followed by a decrease during H 2 S peak loads. Six VOCs and odor concentration increased during the introduction of an H 2 S peak; however no correlation was observed between H 2 S and odor concentration. The increase in odor concentration resulted from the increase in the concentration of six aromatic VOCs, which had their removal slowed down during a 100-ppmv H 2 S peak. Activated sludge diffusion provides effective H 2 S removal with minimal affect on odor emissions. Water Environ. Res., 78, 842 (2006).

A combination of design configurations (tall vs. short aeration tanks and coarse vs. fine air diffusion) was tested at pilot scale for optimum H 2 S removal by activated sludge diffusion (ASD). The effects of H 2 S diffusion on the wastewater treatment performance, in terms of biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS) and ammonia (NH 3 ) removal, were also evaluated, in eight four-week trials. The results showed that although all combinations of configurations gave 85 − 100% removal of up to 25 ppmv H 2 S, a combination of fine bubble diffusers in tall or short tanks gave the best results with regard to H 2 S removal (94 and 99%, respectively). This suggests that aeration tank height has no impact on H 2 S removal when using fine bubble diffusers. However, tank height was important in the configurations with coarse bubble diffusers, with 85% removal being obtained for a short tank and 99% for a tall tank configuration. The configuration with fine bubble diffusers in short tanks gave best treatment performance (75, 72 and 80% mean removal rates for BOD, COD and NH 3 , respectively, at the highest dosing period, compared with 72, 71 and 68% for the control). A brief suspension in nitrification was observed soon after commencement of ASD of H 2 S in the configuration with fine bubble diffusers in tall tanks and that with coarse bubble diffusers in short tanks. The configuration with fine bubble diffusers in short tanks also gave the best mixed liquor suspended solids (MLSS) maintenance and was therefore chosen for further studies to investigate removal of H 2 S peaks of up to 150 ppmv. These studies showed no effects of H 2 S diffusion on the wastewater treatment performance, when the plant was fully stabilized and the microbial biomass was well acclimated.

The increasingly higher costs of fossil fuel are pushing water companies to seek alternative sources of energy such as methane from anaerobic digestion of sludges. Anaerobic digestion can be hindered by several factors including accumulation of grit and rag inside the digester. Periodical clean-out of anaerobic digesters is a common practice in the Water Industry, but very little is known on the nature of the solid material in digester contents. This paper presents the composition of grit removed from a mesophilic anaerobic digester during clean-out operations. Grit and rag accounted for 40% of digester effective capacity but it was impossible to determine the nature of the constituents in the rag due to the large size of debris. However, grit analysis showed that 64% was of inorganic nature while 36 % was organic. It was found that fine grit particles (size between 425 and 1180 μm) represented the largest proportion, accounting for 67% of the total weight of grit. Accumulation of fine grit material could be attributed to inefficient degritting of digester feed, incomplete mixing of the digester content, as well as precipitation of heavy metals inside the digester. Fe3O4 was the main inorganic compound in the grit accounting for 66% of the inorganic fraction of the grit, unsurprisingly as this site uses iron salts for phosphorus removal. These results suggest that optimistaion of chemical dosage in secondary treatment and adequate screening of the digester feed could substantially improve the performance of mesophilic anaerobic digesters.