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Changes in aerobic digester performance with the use of methanol for Biological Nutrient Removal (BNR) in a full-scale Sequencing Batch Reactor (SBR) Koh, Jeff Jae Hong

Abstract

The primary goal of this research was to determine whether the use of methanol in the Sequencing Batch Reactors (SBR) in the District of Kent Wastewater Treatment Plant affected the aerobic digester performance. This was achieved by monitoring four key performance areas of sludge digestion: solids reduction performance; nutrient balance - influx, accumulation, and re-release; stability of digested sludge; and dewaterability of digested sludge. The secondary objective of this project was to investigate general performance issues surrounding the biosolids handling units of the District of Kent Wastewater Treatment Plant in response to the expected increase in incoming flow in the future. Two parallel process trains, Experimental and Control, consisted mainly of the two SBRs and the two aerobic digesters were established to provide means for direct comparison of the effect of methanol. Methanol was injected into the Experimental SBR at dosages of 27, 54 and 81 L/day. The first two dosages were run for one month each and the third for two months. In addition, an extra month of no methanol injection was studied. The methanol injection into the Experimental SBR resulted in a significant increase in the solids level in the SBRs, and the sludge wasting to the Experimental digester was increased accordingly by the plant operators. As the methanol dosage increased from 27 L/day to 54 L/day and finally to 81 L/day, the sludge wasting escalated as well. As the wasting kept increasing, so did the strain on the coarse bubble diffusers to supply sufficient oxygen; when the methanol dosage was at 81 L/day, the dissolved oxygen level in the digesters became inadequate for proper aerobic digestion. The resulting volatile suspended solids reduction was relatively low at 22 and 20% for Control and Experimental digesters, respectively. The total nitrogen influx into the Experimental digester followed that of the volume of SBR waste sludge; as the nitrogen content in the biomass of the two SBR waste sludges was very similar, except when methanol was first introduced and when it was discontinued. The nitrogen accumulation in the digesters was not a concern, as the total nitrogen present in the digesters was largely controllable by the adjusting the wasting of digester sludge. The increased solids, resulting in short sludge ages, and induced low dissolved oxygen levels, were found to limit hydrolysis of organic nitrogen into soluble forms, and therefore limited the release of ammonia back into the SBR, via the decant. The Experimental SBR waste sludges contained much more organic phosphorus in the biomass than the Control SBR waste sludge, but contained less orthophosphate in the bulk liquid. Overall, the total phosphorus influx was greater in the Experimental digester, due to the increased wasting. Accumulation of total phosphorus was also controlled largely by adjusting the digester sludge wasting. The increase in incoming total phosphorus in the Experimental digester, resulted in significant amounts of total phosphorus being rereleased and re-entering the SBR via decant and filtrate. The amount of re-release was high enough to raise the incoming total phosphorus load into the SBR by up to 39%. The specific oxygen uptake rate (SOUR) and volatile suspended solids reduction was used to determine sludge stability. The specific oxygen uptake rate (SOUR) showed that it was affected by the sludge age of the samples. o sludge sample taken during the course of the experiment met the required SOUR value of 1.5 mgO2/1000mgVSShr. With respect to the effect of methanol injection on the sludge stability, the methanol addition to the Experimental SBR (causing shorter sludge ages in the Experimental digester) was found to worsen the stability of the resulting digested sludge. Capillary suction time (CST) test was used exclusively to determine sludge dewaterability. The methanol injection into the Experimental SBR appeared to have no effect on the dewaterability of the digested sludge, as samples from both sides had very similar CST results. It was found that the dewaterability was affected not only by the total suspended solids concentration of the digested sludge, but even more by the temperature of the digested sludge, that controls the filtrate viscosity. Overall, there were no direct effects of the methanol addition to the sludge digestion process, but the increase in sludge wasting to the digester resulted in a detrimental affect on all parts of the digester performance that were monitored, except nitrogen re-release and sludge dewaterability. Filamentous microorganisms did cause sludge bulking in both digesters during the experiment, and Nocardia spp., was identified as the filamentous microorganism most common in the digester sludge. Although there are many factors that could cause the excessive growth of filamentous microorganisms, three possible reasons were identified for this plant: low dissolved oxygen in digester; long sludge age; and low food-to-microorganism ratio (F/M ratio).

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