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Denitrification in low loaded trickling filters
Expanded Title:Within South Africa, at least 130 municipal wastewater treatment works, and another 50 at government institutions, have trickling filters. Trickling filters are employed either as part of a process, or as the sole biological treatment process. These trickling filters are not only found at small towns or remote rural settlements, but are often part of large treatment works, such as Rooiwal Northern Works (220 Ml/d), Olifantsfontein (105 Ml/d), Daspoort (55 Ml/d) and Paarl (25 Ml/d). Amidst current concerns of under-investment in wastewater treatment infrastructure, always in competition with other services for funding, existing trickling filters deserve more attention. This study compromised 3 parts: a) An investigation of the historical data for the Daspoort trickling filters to understand N removal b) Changes to operating parameters (arm rotation and recycle ratios) to evaluate if the system could be improved, and c) Identification of the mechanisms and microbiological processes that play important roles in the nitrogen removal efficacy of the Daspoort trickling filters. The full scale trickling filter experiments confirmed the historic data for the Daspoort Eastern Works in two respects: 1. Nitrogen removal over the trickling filters is very good, with removal efficiencies up to 70%. 2. There is often not enough COD removed to account for the good nitrogen removal in terms of ordinary heterotrophic denitrification. Aside from the observations of good nitrogen removal, it was not really possible to identify operational parameters that led to this performance. None of the experiments on effluent recycle, or distribution arm rotation speeds, improved effluent concentrations significantly, or consistently. It must therefore be concluded that the old trickling filters at Daspoort operated at their optimum already, regardless of this work. Bacteria closely related to anammox bacteria were detected through direct molecular screening of samples from the trickling filters of the Daspoort WWTW. Batch reactors filled with humus sludge demonstrated anaerobic ammonium removal with concomitant nitrite reduction, at a stoichiometric relationship close to that which characterizes known anammox bacteria. However, batch reaction rates decreased progressively over the successive experimental periods, indicating decay of the anammox-like process without growth during batch experiments. With the above as background, it is evident that nitrogen removal over Daspoort trickling filters is not only a function of conventional heterotrophic denitrification, but an anaerobic ammonium oxidation process also plays an important role. A conceptual biofilm process reaction model is developed and the kinetic definitions of the model allow for competition between: • all nitrifiers and ordinary heterotrophs for oxygen, • nitrite oxidisers, denitrifying heterotrophs and anaerobic ammonium oxidisers for nitrite, and • heterotrophs using nitrate, nitrite or oxygen as electron acceptor. The mathematical model of Hao et al. (2001) was integrated into the biofilm compartment of the AQUASIM software. The bulk volume of the biofilm reactor (trickling filter) could be divided into sub-reactors to allow for differentiation of substance concentration in the bulk liquid, as well as for different biofilm thicknesses. An unconfined reactor could be used for the biofilm compartment, which is further specified by no diffusive mass transport of solids and by no change in porosity. A model this defined would be a good biofilm model. However, such a strict definition of the entire system as biofilm model is not consistent with the reality and complexity of the process. Higher organisms such as insects, snails, worms (including earthworms) and crustaceans (including barnacles) seem to thrive in the low loaded intermittently fed ecosystem of the Daspoort trickling filters. Their food would include bacteria and no doubt yeasts, and possibly some of the other higher organisms, which would to a large extent be converted to CO2. The relative mass of these organisms hasn’t been quantified, as distinction from the rest of the biomass complex/matrix is a challenge. Because of their performance and proven potential low loaded trickling filters should be considered as both most sustainable and most appropriate technology in the right context
Date Published:01/02/2014
Document Type:Research Report
Document Subjects:Wastewater Management - Domestic, Wastewater Management - Sewers
Document Keywords:Municipality
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:1825/1/14
ISBN No:978-1-4312-0502-8
Authors:Wilsenach JA; Burke L; Radebe BV; Mashego MR; Stone W; Mouton M
Project No:K5/1825
Originator:WRC
Organizations:CSIR Natural Resources and the Environment
Document Size:2 719 KB
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