The production and biotransformational changes of soluble microbial products (SMPs) and its effects on anaerobic wastewater treatment

• Main Author: Soh, Annie Yan Ni
• Other Authors: Richard D. Webster
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/140429
• Institution: Nanyang Technological University

Effluent quality has been an important factor in the regulatory requirement for discharge of effluent from wastewater treatment plants. Characterization of effluents from both aerobic and anaerobic processes has shown that soluble microbial products (SMP) produced by microorganisms during biological treatment constitute the major proportion of the residual chemical oxygen demand (COD), and this can be up to 100%. Over the past twenty years, advancements in SMP analysis have allowed for their chemical characterization with increasing specificity, however, these advances are still not widely used in the study of anaerobic wastewater treatment systems. Hence, this study aims to examine: the biotransformational changes in soluble microbial products (SMPs), and the trends in their formation and disappearance during anaerobic wastewater treatment; the effects of feed macronutrients on the SMPs produced and their effects on membrane fouling, and; finally, evaluating SMP production in full- scale wastewater treatment plants. An anaerobic baffled reactor (ABR) was used because this type of reactor “splits” the biological reactions (trophic groups) into their constituent parts, as opposed to a continuously stirred tank reactor (CSTR), and hence enables us to monitor the production and degradation/ biotransformation of the SMPs down the reactor over time. The samples collected were extracted by sequential solid phase extraction (SPE) and liquid-liquid extraction (LLE), followed by untargeted analysis using gas chromatography coupled mass spectrometry (GC-MS) and liquid chromatography coupled tandem quadruopole time-of-flight (LC-Q-ToF). The first part of the study on the biotransformational changes of SMPs found the net production of alkanes and alkenes during the pseudo steady-state operation of an anaerobic baffled reactor set at a 24-hour hydraulic retention time (HRT). Previous work reported that the ‘recalcitrant’ phthalate esters are degradable, and a literature search found earlier studies proving that they are products of marine algae. This again showed that such ‘recalcitrant’ microbial products are degradable but are dependent on reactor time (HRT) and conditions (solids retention time-SRT). In terms of the impact of the change in carbon and nitrogen source (macronutrients) in the bioreactor feed on the production of SMPs, the results obtained do not show any obvious changes in the physical parameters routinely monitored for reactor performance (such as COD removal, and volatile fatty acid - VFA production). However, the chemical analysis using chromatographic methods coupled to mass spectrometry revealed underlying biochemical changes over a period of 24 hours after the change in the macronutrients in the feed. One significant finding is that a switch from an organic to an inorganic nitrogen source induced an inhibitory effect on glucose metabolism shown by an increase in the quantity of aliphatic aldehydes produced, which are reaction intermediates of the glucose metabolic pathway, and the ‘disappearance’ of alkanes. A considerable amount of work in the past which has examined fouling in membrane bioreactors were conducted in aerobic systems, but limited information has been available on anaerobic systems. The foulants were characterised in the past using liquid chromatography coupled oraganic carbon detection and organic nitrogen detection (LC-OCD-OND) and excitation- emission matrix (EEM), and these only showed the presence of polysaccharides and proteins. This lack of information led to the detailed chemical characterization of membrane foulants in this study, which has not been reported before. The untargeted scanning using LC-Q-ToF of the extracted membrane foulants found that the compounds detected were in the range of 200 – 799 Da, other than colloids and larger sized particles. A majority of compounds in the foulant were identified as fatty acids and their conjugates, and steroids and their derivatives. Finally, we reported on the characterisation of SMPs in the samples collected at various points of the full-scale industrial wastewater treatment plant, which is an area with limited information in the literature. The chemical characterisation showed some degree of similarity to the SMPs detected during the transient states of the lab-scale bioreactor operating on synthetic feed, while there was a small number of similar compounds primarily comprised of long- chain alkanes and phthalates.