Influence of cyclic feeding on high strength wastewater treatment by activated sludge
The overall efficiency of an activated sludge wastewater treatment system depends both on the ability of the activated sludge culture to remove the soluble organic pollution and on the efficiency of the separation of the treated water from the biomass. In the treatment of a high-strength wastewater (COD > 1.5 kg .m-3), ultrafiltration or crossflow filtration systems can sometimes substitute for the final clarifier, the separation of the sludge from the effluent water being very good regardless of changes in sludge settleability. Because of the high pollutant concentrations, the biomass must be more concentrated in the bioreactor than in the usual aerobic processes (10 to 20 kg.m-3 rather than 4 kg.m-3), whatever the separation system (settler or filtration system). Thus, with this biomass concentration, the volumetric loading rate can be increased while the organic loading rate is maintained at a low level. The use of a clarifier is preferable because it is cheaper than ultrafiltration or crossflow filtration systems, which have very high investment and operation costs, on the other hand, and some fouling problems, on the other hand. However, its use requires a very good biomass settleability. Results from the literature show that there is no general relation between sludge settleability and such wastewater treatment operating parameters as dissolved oxygen concentration in the aeration basin, organic loading rates, pH, or low relative influent nitrogen and phosphorus contents. On the other hand, the separation ability of the clarifier depends on the hydraulic configuration of the aerated reactor: biomass settleability can be controlled by completely stirred reactors in series, plug-flow reactors or intermittently-fed systems. Indeed, this spatial or temporal staggering should allow microorganisms to consume their accumulated substrate and thus restore their accumulation capacity. In the case of high-strength wastewater treatment, temporal staggering is better than spatial staggering. Indeed, higher organic loading rates require a higher number of reactors. Thus, the COD concentration significantly increases in the first reactors of the cascade and some toxic or inhibitory problems could appear. For this reason, a temporal staggering was chosen. Biomass settleability is linked to the extracellular polymers that are the third component of the sludge flocs, after the cells and the water. These polymers are composed largely of microbial exopolysaccharides. Measurements of exopolysaccharides could be used to characterize sludge settleability. A too low level of exopolysaccharides indicates a sludge defloculation problem, whereas a too high level of exopolysaccharides means that sludge settleability could be hindered because of an excessive growth of filamentous organisms. The purpose of the present work is to show that the use of only one completely stirred reactor, fed in a cyclic way with permanent oxygenation and return sludge flow, leads to good sludge settleability and effluent quality. The experiments have been performed with a laboratory pilot-plant composed of a 11.3 litre bubble column, which is a completely stirred reactor, and a secondary settling tank of 14.8 litres. The reactor is fed with an equilibrated synthetic substrate consisting mainly of meat extract, saccharose and ethanol (COD:N:P=100:5:1 and 1 kg COD=1 kg meat extract + 0.444 kg saccharose + 0.2 kg ethanol). The feeding cycle is a set of consecutive periods of feeding and starvation. For example, during a 1h/2h feeding cycle, the substrate feeding is continuous during 1 hour and stopped during 2 hours. The sludge comes from the Nancy-Maxéville wastewater treatment plant and its acclimation to the synthetic substrate begins with an aeration without feeding during 12 hours. Afterwards, the experimental system is fed during 1 hour every 2 hours with the substrate (first concentration of COD=0.1 kg.m-3). Then, the inlet COD is increased by steps of 0.1 kg.m-3 every day until the required concentration is reached. Once the biomass is acclimated to the influent and sufficiently concentrated, the experiment can begin. In order to show the positive influence of cyclic feeding on wastewater treatment quality, we have carried out one experiment with continuous feeding and two experiments with cyclic feeding (see Table 1). For these experiments, the volumetric loading rate is about 4 kg COD.m-3.d-1. With an appropriate feeding cycle, the settleability of the sludge and the effluent quality remain good: the diluted sludge volume index is 56 cm3.g-1 and 96% of the inlet COD is removed. Thus, the biomass concentration remains high in the reactor (19 kg.m-3). This is not the case with a continuous feeding where the biomass settleability rapidly deteriorates. The principal advantage of sequential feeding is the maintenance of good sludge settleability and a high biomass concentration in the aeration basin. Moreover, during the short feeding cycles, the variations of the concentrations of COD, polysaccharides in the bulk phase and extracellular polysaccharides in the microbial aggregates are very low. This augurs well for system stability. In contrast, monitoring of a long cycle (24 h of feeding and 24 h of starvation) shows great variations in the concentrations of COD, polysaccharides and extracellular polysaccharides in the system. When the feeding is stopped, the COD and polysaccharide concentrations in the bulk phase increase whereas the exopolysaccharide concentration in sludge flocs decreases. Reciprocally, after the feeding is begun, the COD and polysaccharide concentrations in the bulk phase decrease, and then remain constant, whereas the exopolysaccharide concentration increases. As 1 kg of measured polysaccharides represents 0.9 kg COD, and as in the bulk phase the COD concentration is equivalent to 4 times the polysaccharide concentration, it seems that microbial products other than polysaccharides are released in the bulk phase during starvation, these products being slowly biodegradable. Extracellular polysaccharides in the microbial aggregates are formed during the feeding phase.