Quantification of EPS and the role PSD and EPS play on dewaterability of THP digested sludge
Molokwu, O.1, Masse, A-L.1, Gavory, M.2 and Moreira, L.2, 1 Thames Water, UK, 2 ENSCR, France
(free)
Abstract:
Several Thames water sites have being experiencing inconsistent or erratic feeding regimes due to irregular feeding controls. This phenomenon is believed to have led to shock loads or starvation of the anaerobic bacteria leading to the possible formation of extracellular polymeric substances (EPS). This paper looks to analyse the particle size distribution (PSD) of the various sludge types, quantify the amount of EPS using a microscope and observe the effects of EPS on dewaterability and polymer consumption of digested sludge. To carry out this experiment, digester feed and digested sludge were sampled from 6 THP sites weekly. Two lab scale digesters (chemostats) were also setup to observe the effects of erratic feeding on the digester. All sludge from this experiment were analysed for PSD, EPS, dry solids (DS) content and polymer consumption. Results showed that there was no significant difference in PSD when correlated with DS content for all tested sites. It was also established that sites with lesser EPS area showed higher DS content and lower polymer consumption when compared to sites with higher EPS area.
Introduction:
The digesters at site B and C have been performing according to Thames waters asset standards in regards to key performance parameters such as biogas yield, pH, VFA etc. The exceptional concerns have been the digested sludge dewaterability on both sites where cake DS (%) target of 32% and 33% for sites B and C respectively and a polymer consumption target of 16kg/TDS for each site have not yet been achieved. One of the hypotheses thought to be responsible for the non-achievement of these set targets is the suspected presence of more EPS in both of the above mentioned sites (site B and C). It is thought that the inconsistent or erratic feeding regimes due to irregular feeding controls on both sites could be responsible for EPS production. It is also thought that the particle size distribution (PSD) in sludge following THP has an influence on the dewaterability of digested sludge. According to (Karr & Keinath 1978), over the course of sludge treatment, the average particle size decreases. Internal studies at Thames water has shown that the feed to THP has larger particle sizes when compared to hydrolysed sludge (digester feed), which in turn has larger particle size when compared to digested sludge. This is explained by the fact that in the course of hydrolysis, the structure of the cell is impacted by the high pressure and temperature. Furthermore, during digestion, bacteria further breaks down cells, thus reducing the particle size even more. There are a few studies available in literature about the impact of particle size distribution on dewatering. Studies by (Karr & Keinath 1978) seemed to indicate that the supracolloidal solids (1-100μm) are the ones having the main impact on dewatering, with a high proportion of supracolloidal material leading to a poorer dewaterability due to filter blinding.
In recent years, the production of EPS by bacteria has been receiving much attention because of their effects on properties and functions of microbial aggregates in biological wastewater treatments. Various literatures have defined EPS as a complex high-molecular-weight mixture of polymers excreted by bacteria, consisting mainly of polysaccharides and proteins, but also of humic acids, uronic acids, nucleic acids, lipids etc (Guo-Ping, et al., 2010). They are also responsible for the structural and functional integrity of the aggregates (Yahui, et al.,2017). According to (Guo-Ping, et al., 2010), the presence of many functional groups – such as carboxyl, phosphoric, phenolic and hydroxyl groups are responsible for the negative charges and very high binding capacity observed in EPS. These biopolymers can also complex with heavy metals and adsorb organic pollutants.
In addition, the binding between EPS and divalent cations, such as Ca2+ and Mg2+, has been identified to be one of the main intermolecular interactions in maintaining the microbial aggregate structure. This bond also plays a significant role in digested sludge dewaterability (Higgins, et al.,2017). In regards to structure, we commonly find two parts of EPS; the soluble EPS and the bound EPS. EPS in activated sludge particles are heterogeneous, there is most likely a dynamic double-layered structure of loosely bound EPS (LBEPS) encircling the tightly bound EPS (TB-EPS) (Yahui, et al.,2017). This bound EPS have been identified as a gel like material that is able to hold a lot of water inside which keeps the environment around the cell hydrated. This also means that releasing water from this gel will be difficult (Rus, 2017). Hence, the EPS double layer seems to hamper the digested sludge dewatering and may be part of the reason for the high polymer demand and low cake dry solids content seen onsite.
