A critical element to the successful operation of activated sludge systems is efficient solid liquid separation achieved by bioflocculation. Bioflocculation refers to the process of microbial aggregation to form activated sludge flocs, dependent on the interaction of exocellular polymeric substances (EPS) to form the matrix that holds microbes, other organics and inorganic particles in a flocculent mass. Numerous factors affect bioflocculation; two key parameters are the Solid Retention Time (SRT) and the substrate loading rate. The latter is related to the two basic designs in activated sludge bioreactor configurations: the Plug Flow Reactor (PFR) and the Completely Stirred Tank Reactor (CSTR). PFR systems have a high substrate loading rate, whereas CSTRs have a low substrate loading rate. Research has shown that the PFR configurations produce better sludge quality, in terms of settleability and dewaterability, and subsequently better effluent quality than CSTR systems.

In this experiment, the effect of SRT and substrate loading rate on activated sludge was investigated using bench scale SBRs. PFR and CSTR configurations were simulated by adjusting the fill period to be shorter or longer respectively. A series of SBRs were operated, each with an operating volume of 6L, to obtain data for PFR (fast feed) versus CSTR (slow feed) configurations at 10 Day, 5 Day and 2 Day SRTs. Effluent quality was monitored by measuring effluent TSS, VSS, total and soluble COD and soluble biopolymers. Sludge quality was monitored for the aerobic phase by measuring total and suspended solids, total and suspended volatile solids, Sludge Volume Index (SVI), Capillary Suction Time (CST) and Zeta Potential. Anaerobic digestibility was measured for the sludge produced in these systems by measuring gas production, similar to estimating biogenic methane potential (BMP) and determining short term odor productions, specifically Total Volatile Organic Sulfur Compounds (TVOSCs).

As expected the change in feeding pattern and SRTs affected the effluent and sludge quality during the aerobic operation phase. Effluent quality was found to be better for the fast feed system at all SRTs, with all monitored parameters being of similar or significantly lower concentration than for the slow feed system. In terms of sludge quality, the fast feed system was found to retain more of its biomass in solution, indicating better flocculation and settleability in this system. COD was given a lower rank as an effluent quality indicator, since the 5 Day and 2 Day SRT datasets did not correlate well with other datasets, specifically effluent TSS and biopolymers. The data was included because it is believed that the trends were accurate representations of fast versus slow feed system behavior. The trends were comparable to those of effluent TSS and solution biopolymer datasets.

In terms of anaerobic digestion potential, the fast feed sludge exhibited greater volumetric gas production per gram of solid at the 5 and 2 Day SRTs. Gas production was similar for both systems at the 10 Day SRT. Total and Volatile Solid reduction were however found to be higher for the slow feed sludge than for the fast feed. This may indicate higher gas and potential odor production per gram of solid degraded for the fast feed sludge. This theory is supported by the odor analyses, which revealed that the fast feed sludge had a higher TVOSC production at each SRT. This was related to the higher protein content of the sludge, indicated by the effluent biopolymers being much higher in protein content than carbohydrates. Shearing, which is part of the solids handling process at most plants, releases these proteins and makes them bioavailable, allowing them to be oxidized to produce TVOSCs and hence higher odors.

In conclusion it was found that the fast feed effluent and sludge quality appeared to be overall better at each SRT simulated; the higher TVOSC content may indicate a problem with solids handling, but research has shown that these can be overcome with the addition of iron. Additionally, both systems, the fast and slow feed systems operated better at longer SRTs, with the fast feed system performing better in all cases. The difference was not completely significant in all cases and this is attributed to being a by-product of operating at the optimal M:D salt ratio.

This project has strength in terms of its potential for large scale applications. SRT is the considered the most important design parameter and one of the more complicated parameters to manipulate due to its widespread effect on reactor behavior, specifically sludge and effluent quality. Additionally, the fast feed versus slow feed concept is one that has been gaining significant interest, since bioreactor configuration impacts the effluent and sludge quality. Feed configurations have been investigated more frequently within the past decade. The novel approach taken by this project is that it combines these two parameters, both of which are important to large scale plants, both industrial and municipal.