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Recent advances in activated sludge facilities operations utilize sludge recycle flow rate control to minimize adverse effects on the clarifier during peak solids loading situations. Although this control action is directed at the thickening function, there has been speculation that the elevated recycle rates may be responsible for an increased effluent solids concentration. To evaluate the significance of recycle rate to the steady-state effluent suspended solids concentration, identical side-by-side settling columns were operated with recycle rates spanning the range of normal practice. The studies were conducted so that each side-by-side unit received identical activated sludge feed slurries. Replicate experimental units operating at identical recycle rates and receiving identical feed slurries were used to determine experimental error. The experimental design facilitated a statistical determination of the significance of the effect of recycle flow rate. An analysis of variance procedure was pursued, with the conclusion that recycle rate does, in fact, influence suspended solids removal. Ramifications for facilities design and operation is also reviewed.

Utilization of the activated sludge process is widespread although many of the mechanisms involved are still relatively misunderstood. Incorporation of nitrate respiration (denitrification) into the activated sludge process can have many advantages, but little is known about microbial growth and substrate removal when nitrate respiration is employed. The primary objective of this study was to evaluate and compare microbial growth and biokinetic coefficients in an aerobic and an anoxic (anaerobic with nitrate as the terminal electron acceptor) activated sludge process. Two bench-scale continuous flow reactors were operated over a range of mean cell residence times with organic carbon as the limiting nutrient. Alkalinity changes were monitored and compared with theory. Engineering applications of the results were discussed. The maximum microbial yield and endogenous decay coefficient were lower, and the maximum substrate utilization rate was higher for nitrate versus oxygen respiration. Alkalinity production during denitrification was very near the theoretical stoichiometric value of 3.57 mg as CaCO₃ per mg NO₃⁻-N denitrified. It was concluded that single-sludge systems incorporating organics removal, nitrification, and denitrification can potentially achieve a high degree of nitrogen and organic carbon removal at lower cost than a similar size system incorporating organics removal and nitrification only. Aeration energy savings and reduced sludge production obtained by the utilization of nitrate respiration in single-sludge systems should result in significant cost savings. / M.S.

The cost of handling and disposal of sewage sludge is a major expense in sewage treatment. Often half of the treatment plant expense is in the purchase and maintenance of sludge handling equipment. The sludge problem becomes particularly acute with activated sludge processes because of the large volumes of waste activated sludge produced and the nature of the activated sludge itself. The object of this investigation was to conduct laboratory scale investigations of the effects of aerobic digestion on activated sludge dewatering and to attempt to determine the chemical and biological mechanisms affecting sludge filterability. The results of this investigation showed that aerobic digestion can have a considerable effect on sludge filterability. The filterability of all sludges was improved with moderate aeration at 20° C. The degree of improvement depends on the nature of the fresh sludge, the rate of aeration during digestion, the temperature during digestion, and the time of digestion with the optimum time of digestion between four to six days. The mechanism of filtration improvement during aerobic digestion appears to be biological in nature. Biologically produced extracellular polymeric substances are secreted by or exposed on the surface of bacteria during the endogenous phase of metabolism. These polymers cause coagulation and bridging between sludge particles, thus affecting the filtering properties. This phenomenon occurs only when oxygen is present in the system. However, the organisms responsible can survive up to four days without the artificial addition of oxygen. A study of various operating parameters in aerobic digestion leads to some further conclusions regarding sludge filtration. Factors which have a detrimental effect on filtration are: lack of dissolved oxygen, excessive mixing, low temperature during digestion, and chlorination of sludge before filtering. Factors which do not appear to have a significant effect on filtration are: differences in dissolved oxygen concentrations above two mg per liter, and the maintenance of a neutral pH in the digester. / Ph. D.

The extent of release and precipitation of phosphorus stored in the poly-phosphate granules of microorganisms present in anaerobic digesters sequencing a biological phosphorus removal (BPR) activated sludge system was examined. The research was conducted at the York River Wastewaster Treatment plant which was converted from a conventional system to a A/O process for biological phosphorus removal. The primary and secondary anaerobic digesters each had a solids retention time of approximately 120 days. The results indicated that at least 60 percent of the phosphorus in the poly-phosphate granules would be solubilized during endogenous decay and digestion under anaerobic conditions. This is accompanied by the release of potassium, magnesium and limited amounts of calcium. The release of magnesium and phosphorus in the high ammonium containing environment of the primary digester results in the precipitation of about 1000 mg/L of struvite (MgNH₄PQ₄H₂O), a relatively adhesive and kinetically fast precipitate. Struvite can form on floes of biomass inside the primary digester, on walls, inside pipes, and in areas affected by lower temperatures and higher pH. The phosphorus level in the secondary digester effluent is controlled by the kinetics of formation of competitive precipitates like struvite, calcium-phosphorus compounds and vivianite (Fe₃(PO₄)₂). The formation of vivianite is limited by the competitive precipitation of siderite (FeCO₃ ). The extent to which the compounds are precipitated depends on the solids retention times, ionic strengths, size and amorphous nature of the precipitates and the substitution of foreign ions in the crystal lattice. The effective solubility products of these precipitates, as applicable to anaerobic digesters, were determined. / M.S.

A viable sludge management alternative is land application of waste sludge. However, it is necessary to implement efficient monitoring and analysis of land applied sludges in order to assess potential health risks associated with this means of disposal. The State of Virginia is considering a proposal that requires land-applied wastewater sludges to undergo analysis by EPA's Toxicity Characteristic Leaching Procedure (TCLP) (EPA, 1986b) to determine if the sludge exhibits hazardous characteristics, which preclude land application as a management alternative. The method currently used for the analysis of hazardous wastes is the Extraction Procedure (EP) Toxicity Test. Both of these test methods analyze for trace organic chemicals and heavy metals. However, the TCLP is designed to analyze for volatile organic chemicals to a greater extent than the EP. Because of the added complexity and the current expense of the TCLP, the State is concerned that the TCLP may not be warranted for the analysis of trace organic chemicals in land-applied sludges. This research was designed to compare the abilities of the EP and TCLP for the analysis of trace organic chemicals in wastewater sludges. Samples from three municipal wastewater treatment plants that utilize secondary biological treatment, aerobic digestion and land-apply waste sludge were evaluated by both EP and TCLP methods. Both tests utilize a weak acid extraction to remove organic chemicals from the wastewater sample. The weak acid extract was subjected to liquid-liquid extraction (EPA Method 625) to partition and concentrate the organic chemicals into methylene chloride; this methylene chloride extract was then subjected to GC and GC/MS for quantitative analysis and qualitative identification of targeted and nontargeted organic chemicals. In order to assess recovery and extractability efficiencies of each test, surrogate standards were added prior to the test procedure. These standards were bromoform, 1-chlorooctane, DDT, ethylene dibromide (EDB, a volatile fungicide), fusarex (tetrachloronitrobenzene), and heptachlor. Control samples were run for both EP and TCLP, in addition to a sludge samples with no surrogates added. Analysis indicated that both the TCLP and EP tests showed high variability for the recovery of the sludge surrogates. The recoveries of the surrogate standards were low and varied between zero and 30 percent depending on the standard and the matrix. Surrogate recoveries were evaluated with respect to various physical/ chemical properties of the individual standard, the sample site, and the test method utilized. Although the TCLP recovered the volatile surrogate standards only slightly better than the EP, there was no statistically significant difference between the TCLP and EP for the recovery of the non- and semi-volatile surrogate standards. Specific trace organic chemicals identified in the sludges included dimethylpentanol, dichlorodimethoxybenzene, 4-methylphenol, and tetrabutylphenol. Other chemicals, such as contaminants and artifacts resulting from laboratory processing and background contamination in the reagents, were also identified in the blank control samples as well as the sludge samples. / Master of Science

The purpose of this investigation was to determine what effect agitation had upon the biochemical oxygen demand of sewage. In order to determine the effect of agitation, the composition of the sewage, the seed source, the temperature and the dilutions were held constant. / Master of Science