Removal of metals for improvement of sludge quality, adsorption to primary sludge during primary settlement

• The primary and secondary sludge from a wastewater treatment plant are generally mixed and treated combined. Here we introduce an idea for a process concept where the sludge flows are separated and the treatment of primary sludge is modified, with the goal to concentrate micropollutants in primary sludge while nutrients are removed in the secondary (biological) treatment to produce a “bio-sludge” with low metal contents. The example is based on primary settlement and an activated sludge process. In contrast to a conventional process, the sludge flows are as mentioned separated. After anaerobic digestion and dewatering, primary sludge goes through pyrolysis. Biochar produced during pyrolysis is added in pulverized or granulated form to the primary settler. The hypothesis is that biochar will adsorb dissolved metals and thus enhance the metal removal in primary treatment. The biochar should settle with primary sludge, and pyrolysis is repeated. However, to remove metal content from the system some portion of the produced biochar will have to be removed in each cycle. A prerequisite for nutrients to end up in the bio-sludge is that chemical coagulants are not used in primary treatment and that there is no recirculation of sludge from secondary to primary treatment. To the best of the authors knowledge, biochar has not previously been tested as an adsorbent in primary treatment of wastewater. Efficient removal of metals has though been shown in several studies where wastewater was filtrated through biochar in granulated form (Huggins et al., 2016). Further, biochar has been shown to sorb pharmaceuticals from urine without removing nutrients (Solanki & Boyer, 2017). In this paper, results from experimental tests on addition of biochar in the primary settler will be presented. Experiments are made in lab-scale to test the adsorption and settling capacity depending on biochar properties, e.g. particle size, cation exchange capacity. The theoretical dosing requirement in a full scale application and possible biochar yields from pyrolysis of primary sludge are also investigated.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Miljöbioteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Environmental Biotechnology (hsv//eng)

Nyckelord

Biotechnology/Chemical Engineering

bioteknik/kemiteknik

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