<p> Laboratory and field measurements were combined with theory in developing models for the various wastewater treatment processes. Some assumptions were made, both in an attempt to simplify the complex mechanisms encountered and to explain known experiences and observations. </p> <p> Models for a primary settling tank, a secondary settling tank, a generalized aerobic biological sludge treatment scheme, an anaerobic digestion system and several other unit processes have been developed. The generalized aerobic biological sludge treatment scheme is an attempt to describe in one physical package, all the various activated sludge process modifications. </p> <p> The executive system used is GEMCS · together with its library of subroutines, such as CONTLl, SEPAOl, JUNCOl and SETSPI, some of which have been modified slightly. </p> <p> The models have been tested on the Drury Lane and the Skyway, Burlington, Sewage Treatment Plants, and have been found to agree very well with plant data. The former has a conventional activated sludge process, whilst the latter employs the extended aeration modification. </p> <p> An inter-active version has been developed, which requires the minimum knowledge of GEMCS for its use. This can be applied as a training and teaching tool for plant operators and students in the field. </p> / Thesis / Master of Engineering (MEngr)

Yes / To date, mixing design practice in anaerobic digestion has focussed on biogas production, but no adequate consideration has been given to energy efficiency. A coherent, comprehensive and generalized strategy based on computational fluid dynamics (CFD) modelling is proposed to improve mixing efficiency of a full-scale, unconfined gas-mixed digester for wastewater treatment. The model consists of an Euler-Lagrange (EL) model where biogas bubbles are modelled as the Eulerian dispersed phase, and non-Newtonian sludge as the Lagrangian continuous phase. Robustness tests show that mixing predictions are independent of bubble size. The CFD strategy comprises the assessment of different mixing geometries and a range of input gas flow rates. Quantitative results show that simple retrofitting measures are able to achieve a significant improvement in the degree of mixing with reduced mixing times, and consequently recommendations for best mixing geometry and gas flow rate are given. A generalization to a generic digester is discussed in a form that is readily usable by professionals and consultants.

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 Blue Plains Wastewater Treatment Plant in Washington, D.C. is in the process of updating its anaerobic digesters, with a completion date set for 2008. This research was undertaken to aid designers at Blue Plains by evaluating alternative digestion approaches. The technologies applicable to the plant included thermophilic anaerobic digestion, acid/gas phased digestion, and temperature phased anaerobic digestion. To obtain design data, a year long study was conducted using bench scale digestion systems operating at different solids retention times (SRT) and organic loading rates (OLR). The digesters were fed with mixed primary and secondary waste (50/50 by weight) from the Blue Plains wastewater treatment facility. The digesters were operated until they reached steady state as observed by volatile solids reduction (VSR), methane production, and volatile fatty acid (VFA) levels. At this point, samples of digested waste sludge were analyzed for residual biological activity, dewatering properties and headspace organo-sulfur production. Results from the study demonstrated that the TPAD digestion process had the lowest residual biological activity (RBA) after digestion, and that the single stage thermophilic digesters had the highest RBA. Sludge from single stage mesophilic digestion had the highest odor potential, with headspace gas tests generating over 1400 mg organo-sulfur per m3 of headspace gas, while both single stage thermophilic and TPAD systems generated less than 400 mg/m3 at all SRTs studied. / Master of Science

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

Conditioning studies were conducted on several water and wastewater sludges using polymers to determine the effect of high-intensity mixing on floc formation and breakup. Mechanisms acting in particle coagulation were also investigated. A rapid mixing apparatus was used in conditioning the sludge to simulate the high velocity gradients encountered in most dewatering equipment. Anionic and cationic polymers of varying molecular weights and an inorganic coagulant were chosen as the conditioning agents. Dewatering rates were measured by the capillary suction time (CST) apparatus. The effect of rapid mixing on the polymer efficiency was evaluated by premixing the polymer, using the same mixing apparatus, prior to applying it to condition the sludge. Data analyses revealed the great impact the primary floc breakup has on the sludge dewatering rate, although this was shown to be dependent on the nature of the sludge particles. Some minor changes were imparted to the polymer chains by intense mixing but there was no evidence that particle polymer bonds were fractured. The mechanisms of coagulation were observed to be related to sludge type. Overall, successful conditioning of sludges by polymers requires a proper determination of the sludge characteristics, a correct choice of polymer type and dose, and a knowledge of the mixing intensities. / Master of Science

The operation of three continuous flow units as modified activated sludge models using mean cell residence time as a principal operational parameter has led to the following conclusions: 1. Lower mean cell residence times result in higher phosphorus removal efficiencies in biological reactors. 2. Higher mean solids residence times result in higher phosphorus removal efficiencies in strictly chemical reactors. 3. Calcium-phosphate precipitation can occur in continuous flow reactors in a normal activated sludge pH range (7.5-8.5) given sufficient Ca++ concentrations. 4. Calcium-phosphate precipitation can occur in the activated sludge process without hindering COO removal efficiency. 5. The phosphorus sludge content only exceeded the one to two percent range in the biochemical and chemical units, where phosphorus precipitation was the predominate phosphorus removal mechanism. 6. The anaerobiosis and acid stripping procedure, as used in this investigation, is not effective in enhancing either a biological or chemical unit's ability to remove phosphorus. / Master of Science