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This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights: A pilot scale biofilter removed H2S and NH3 in a wastewater treatment plant. The biofilter produced small volume of leachate which contains ammonium sulphate. The ammonium sulphate produced can be harvested for further use. AbstractBiofilters are popular for the removal of odours from gaseous emissions in wastewater treatment plants because of their low capital costs and low energy requirements. In an aerobic environment, the microbes in biofilter oxidize odorous gases like hydrogen sulphide (H2S) and ammonia (NH3) to non-odorous sulphate and nitrate. This paper describes a pilot plant biofilter setup at a local waste water treatment plant (WWTP) which has been in continuous operation for more than 150 days, removes both H2S and NH3 at an average removal efficiency of 91.96% and 100% respectively. Unlike a conventional biofilter, the pH of this biofilter was not adjusted by addition of chemicals or buffers and the H2SO4 produced from the biological conversion of H2S is periodically washed down and allowed to accumulate in a concentrated form at the base of the biofilter. NH3 entering at the base is removed, not by biological oxidation, but by the chemical reaction of ammonium with sulphate to form ammonium sulphate. The ammonium sulphate produced in biofilter is washed down and the volume of leachate produced is less than 0.2mL of leachate/L of reactor/day. Estimated cost savings of converting the current chemical scrubber used at the WWTP to a similar biofilter described in this study is included with this paper.

Large quantities of sludge is generated from different sections of a wastewater treatment plant operation. Sludge can be a solid, semisolid or liquid muddy residual material. Understanding the flow behaviour and rheological properties of sewage sludge at different sections of a wastewater treatment plant (WWTP) is important for the design of pumping system, mixing, hydrodynamics and mass transfer rates of various sludge treatment units, optimization of conditioning dose and for sustainable sludge management. The current article provides a comprehensive review on up to date literature information on rheological behaviour of raw primary sludge, excess activated sludge, thickened excess activated sludge, mixture of raw primary and thickened excess activated sludge (mixed sludge), digested sludge, and biosolid under the influence of different operating parameters and their impacts on process performance. The influences of various process parameters such as solid concentration, temperature, pH, floc particle size, primary to secondary sludge mixing ratio, aging and conditioning agent doses on the rheological behaviour of sludge from different treatment units of WWTPs are critically analysed here. Yield stress was reported to increase with increasing solid concentration for all types of sludge whereas viscosity showed a decreasing trend with decreasing total solid concentration and percentage of thickened excess activated sludge in the mixture. Temperature showed an inverse relationship with yield stress and viscosity. Viscosity was reported to be decreased with decrease in pH. The effect of various conditioning agents on the rheological behaviour of sludge are also discussed here. The applicability and practical significance of various rheological models such as Bingham, Power Law (Ostwald), Herschel-Bulkley, Casson, Sisko, Careau, and Cross models to experimental rheological characteristics of various sludges were presented here. The reported results on various rheological parameters such as shear stress, yield stress, flow index, infinite, zero-rate viscosity, and flow consistency index of different sludge types obtained from the best fitted model were also compiled here. Conclusions have been drawn from the literature reviewed and few suggestions for future research direction are proposed.

Rheological characterization of sludge is known to be an essential tool to optimize flow, mixing and other process parameters in wastewater treatment plants. This study deals with the characterization of thickened excess activated sludge in comparison to raw primary sludge and excess activated sludge. The effects of key parameters (total solid concentration, temperature, and pH) on the rheology and flow behaviour of thickened excess activated sludge were studied. The rheological investigations were carried out for total solid concentration range of 0.9%w/w-3.7%w/w, temperature range of 23°C-55°C, and pH range of 3.6-10.0. Different rheological model equations were fitted to the experimental data. The model equations with better fitting were used to calculate the yield stress, apparent, zerorate, infinite-rate viscosities, flow consistency index, and flow index. The decrease in concentration from 3.7%w/w to 3.1%w/w resulted in drastic reduction of yield stress from 27.6Pa to 11.0Pa, and further reduction of yield stress to 1.3Pa was observed as solid concentration was reduced to 1.3%w/w. The viscosity at higher shear rate (>600s-1) decreased from 0.05Pa.s down to 0.008Pa.s when the total solid concentration was reduced from 3.7% to 0.9%. Yield stress decreased from 20.1Pa down to 8.3Pa for Bingham plastic model when temperature was raised from 25˚C to 55˚C. Activation energy and viscosity also showed decreasing trend with increasing temperature. Yield stress of thickened excess activated sludge increased from a value of 6.0Pa to 8.3Pa when pH was increased from 3.6 to 10.0. The effect of polymer dose on the rheological behaviour of thickening of excess activated sludge was also investigated, and the optimum polymer dosage for enhanced thickener performance was determined to be 1.3kg/ton DS.

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