Abstract
Biological activated carbon (BAC) filters can be used to remove residual total organic carbon (TOC) from greywater after a membrane bioreactor. The two main TOC removal processes are adsorption to the granular activated carbon (GAC) and biological degradation. Biodegradation leads to the growth of microorganisms in the filter bed, which can lead to increased pressure loss over the filter bed. However, the roles of sorption and biodegradation in long-term TOC removal and how they complement each other are unclear. We monitored TOC removal from greywater in a BAC filter installed following a membrane bioreactor over more than 900 days. Removal performance depended on the operational time of the BAC filter, the influent TOC concentration, and in the upper part of the filter on the empty bed contact time (EBCT). Across the overall filter, the EBCT did not significantly influence TOC removal, showing that the filter was sufficiently large for the range of flow rates observed. Analysis of the long-term data revealed the equal importance of sorption and biodegradation over the whole operation period and the whole filter bed. Most of the TOC was removed in the upper part of the filter, where biodegradation was the dominant mechanism. In the lower part of the filter, sorption capacity remained and allowed high influent TOC concentrations to be buffered. The generous filter design with low average filtration rates ensured long-term TOC removal. The only maintenance needed was backwashing, which was required only after more than 800 days of operation. Backwashing effectively reduced the pressure loss but had no significant influence on the effluent water quality. Our study shows that BAC filters are a suitable post-treatment step for the treatment of greywater with highly variable flow and TOC concentrations.
Highlights
Greywater reuse offers substantial water-saving potential with the benefit that the water is available during dry periods (National Academies of Sciences 2016)
Our study shows that Biological activated carbon (BAC) filters are a suitable posttreatment step for the treatment of greywater with highly variable flow and total organic carbon (TOC) concentrations
The extent to which various depth segments in the BAC filter may be at different stages of organics removal has not been studied, and neither has the extent to which biodegradation is beneficial to maintaining sorption capacity
Summary
Greywater reuse offers substantial water-saving potential with the benefit that the water is available during dry periods (National Academies of Sciences 2016). Analysing long-term data and understanding how TOC degradation is influenced by different factors in a greywater treatment system treating real greywater are important. The two main removal mechanisms for organic carbon in biological activated carbon (BAC) filters are adsorption and biodegradation. For fresh granular activated carbon (GAC), adsorption is the main removal mechanism. As the volume of water treated increases, adsorption ca pacity is exhausted, bacteria accumulate in the filter bed, and biodeg radation becomes increasingly important for TOC removal. From a certain point of time on, biodegradation is the main removal mechanism (Simpson 2008). How a BAC filter may be designed to use both adsorption and biodegradation optimally for TOC removal has yet to be clarified
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