Characterisation of Chemical Processes Operating within a Biological Wastewater Treatment Plant

Buck, Andrew (2004). Characterisation of Chemical Processes Operating within a Biological Wastewater Treatment Plant. PhD thesis The Open University.



The activated sludge process used in biological wastewater treatment plants utilises a mixed consortia of microorganisms that break down complex organic compounds into smaller less toxic components. The complex mechanisms and pathways that exist within the biomass have not been studied to any great degree and as a consequence little is known about the specific fate pathways for many organic compounds being metabolised by the microorganisms. Studies carried out using a laboratory biological reactor using biomass taken from a large scale treatment plant have allowed a detailed determination of the various pathways for a range of rubber chemical compounds present in the wastewater from a rubber chemical manufacturing plant. In particular it has been found that concentration plays an important role in the metabolic pathway for many compounds: two different pathways exist depending on the concentration of a given substrate. Furthermore, it has been shown that the presence of other substrates can influence the pathway and metabolites generated during mineralisation of a substrate.

The compound mercaptobenzothiazole (MBT) is an example of a concentration dependent substrate. At low concentrations the MBT is broken down through initial methylation leading to the formation of 2-hydroxybenzothiazole and ultimately catechol. However, at higher concentrations the MBT is oxidised to form the disulphide 2,2'-dithiobisbenzothiazole. This disulphide is removed following oxidative cleavage of the sulphur-sulphur bridge producing a series of acid components including 2-benzothiazolesulphenic and sulphinic acids.

The compound para-hydroxybenzoic acid (PHBA) has been shown to be one of a series of substrates required for the production of enzymes that are part of an overall scheme in which a substance giving a characteristic 'red' colouration to the effluent is produced and/or controlled. It has been shown that an absence of PHBA leads to the loss of a secondary substrate-enzyme complex, which subsequently leads to formation of a substance with a highly coloured chromophore following the reintroduction of the PHBA substrate.

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