Jeannie L. Darby

Ultraviolet disinfection of wastewater: Effect of dose on subsequent photoreactivation

Abstract The objective of this research was to examine the significance of photoreactivation of total coliform for u.v. disinfected secondary effluent. An appropriate methodology for quantifying photoreactivation was identified. Photoreactivation was evaluated in relation to u.v. dose, exposure to photoreactivating light and various water quality parameters. An increase in u.v. dose is extremely valuable in minimizing photoreactivation events. A significant inverse relationship was found between average u.v. dose and photoreactivation of coliform in wastewater at both low and high u.v. doses. Increased levels of suspended solids had the effect of reducing the true u.v. dose reaching targeted organisms, thus reducing DNA damage and increasing subsequent photoreactivation. The dose of photoreactivating light was also observed to be an important parameter in the reactivating process.

Extracellular polyanions in digested sludge: Measurement and relationship to sludge dewaterability

The polyanionic fraction of digested sludge extracellular material was quantified using an in situ dye adsorption method, and the relationships between measured extracellular polyanion (ECPA) concentrations and sludge dewaterability were investigated. Measured ECPA concentrations were 18–30% of the sludge total solids (32–46% of the volatile solids), when expressed as equivalents of sodium alginate. These values may be an overestimate of the true value, but are likely a more accurate estimate than those derived from extraction-based methodologies. The relationship between total ECPA concentration and digested sludge dewaterability was dependent on digester feed composition, suggesting that specific ECPA biochemical characteristics are more significant than total concentration as an influence on dewaterability. The influence of the soluble/colloidal fraction of the total ECPA on digested sludge dewaterability was consistently adverse, but from a full-scale dewatering perspective was insignificant.

Ultrafiltration of wastewater: effects of particles, mode of operation, and backwash effectiveness.

The effects that wastewater quality and mode of operation have on the performance of an asymmetric, hollow fiber, polysulfone, ultrafiltration (UF) membrane with a molecular weight cutoff of 100,000 Daltons were investigated. Performance was assessed through monitoring membrane flux, transmembrane pressure, effluent biochemical oxygen demand, and operational cost of the experimental system while treating filtered secondary, secondary, and filtered primary effluents. Fluxes achieved for filtered secondary (129-173 l/m2 h), secondary (101-158 l/m2 h), and filtered primary (20-41 l/m2 h) effluents were compared to those obtained at three other locations where similar UF systems were operated. A conceptual model of the impact of an insufficient backwash and of operating the UF system at constant flux on membrane performance is presented to explain the differences in fluxes. Employing pre-membrane granular filtration to remove a portion of the problematic particles in secondary effluent prior to UF led to optimal operational conditions. The costs associated with the operation of pre-membrane granular filtration were offset by the increase in production achieved. Although the use of recirculation could increase maintainable flux when treating a concentrated feed (e.g., filtered primary effluent), the associated costs were high. Improved UF performance was found to result from allowing flux to decline naturally, rather than using a constant flux mode of operation. The effluents produced when filtered secondary and secondary effluents were the feeds would be equivalent to an oxidized, coagulated, clarified, and filtered wastewater as per Title 22 California Wastewater Reclamation Criteria.

Factors influencing ultraviolet disinfection performance Part II: association of coliform bacteria with wastewater particles

A technique is described for quantifying the number of particles with embedded coliform bacteria in wastewater samples. This technique was applied to eight different wastewater samples collected from eight significantly different treatment processes (five variations of the activated-sludge process, a trickling filter, an aerated lagoon, and a facultative lagoon). For all treatment process types that did not use chemical coagulants, total number of particles with embedded coliform bacteria correlated linearly with number of residual coliform bacteria following high doses of UV light. Lagoon treatment processes led to the formation of fewer bacteria-associated particles than either the activated-sludge or trickling-filter processes. Chemical coagulants showed a potential to significantly reduce the inactivation fraction of coliform bacteria associated with particles.

Clogging in intermittently dosed sand filters used for wastewater treatment

Clogging in intermittent sand filter (ISF) systems was analyzed using an unsaturated flow model coupled with a reactive transport model. Based on the results of a model sensitivity analysis, several variables were determined to be important in the clogging phenomena observed in ISFs, including hydraulic loading rate, influent chemical oxygen demand (COD) concentration, filter dosing frequency, and time of operation. Several modes of operation were identified that minimize the growth of bacteria at the filter surface. Following the sensitivity analysis, several case studies where ISF clogging was documented were simulated using the model. The results from the case study model simulations were found to be correlated with the total suspended solids loading rate (TSSLR) at the point of clogging. A model was developed that relates biomass development at the surface of ISFs with the TSSLR that can be sustained without clogging. The engineering significance of the model is presented in terms of operational and design considerations.

Intermittent slow sand filtration for preventing diarrhoea among children in Kenyan households using unimproved water sources: randomized controlled trial.

Objective  Measure effectiveness of intermittent slow sand filtration for reducing child diarrhoea among households using unimproved water sources in rural Kenya.

Ultrafiltration as an advanced tertiary treatment process for municipal wastewater

Abstract Treatment of secondary and tertiary municipal wastewater by membrane filtration has been investigated at the University of California, Davis, California, USA, as described in the following paper. Secondary and tertiary wastewater used during this investigation were produced from activated sludge and media filtration processes, respectively. Ultrafiltration, hollow-fiber, polysulfone membranes were used in this investigation for the treatment of secondary and tertiary municipal wastewater. Membrane operating parameters investigated were cross-flow velocity, with circulation and single pass flow, trans-membrane pressure and backflushing methods. Total solids and particle size distribution of secondary and tertiary wastewater were analyzed and correlated with observed membrane performance. Reduction of total solids, biological oxygen demand (BOD), and chemical oxygen demand (COD) of secondary wastewater was also determined after treatment with membrane and media filtration processes.

Extended Triple Layer Modeling of Arsenate and Phosphate Adsorption on a Goethite-based Granular Porous Adsorbent

The extended triple layer model (ETLM), which is consistent with spectroscopic and theoretical molecular evidence, is first systematically tested for its capability to model adsorption of arsenate and phosphate, a strong competitor, on a common goethite-based granular porous adsorptive media (Bayoxide E33 (E33)) in water treatment systems under a wide range of solution conditions. Deprotonated bidentate-binuclear, protonated bidentate-binuclear, and deprotonated monodentate complexes are chosen as surface species for both arsenate and phosphate. The estimated values of the ETLM parameters of arsenate for the adsorbent are close to those for pure goethite minerals previously determined by others. The ETLM predictions for arsenate and phosphate adsorption basically agree with experimental results over a wide range of pH, surface coverage, and solid concentrations. High background electrolyte concentration (i.e., I = 0.1 M), however, was found to strongly impact arsenate and phosphate adsorption on E33 probably because of the porous structure of the adsorbent, which cannot be observed for pure goethite minerals and could not be completely modeled by the ETLM. Prediction of phosphate adsorption isotherms at higher pH were relatively poor, and this may suggest searching for alternative surface species for phosphate. Since adsorption equilibrium constants of major coexisting ions encountered in water treatment systems for goethite minerals have been estimated by others, the application of ETLM theory to this common goethite-based adsorptive media will enable us to understand how those coexisting ions macroscopically and thermodynamically interact with arsenate and phosphate in the environment of adsorptive water treatment system in a way consistent with molecular and spectroscopic evidence.

Association of coliform bacteria with wastewater particles: impact of operational parameters of the activated sludge process.

The fraction of particles with associated coliform bacteria (PAC) in the activated sludge process was evaluated using a 16S rRNA oligonucleotide probe specific to the family Enterobacteriaceae. The PAC was found to decline exponentially with increasing mean cell residence times (MCRTs). The factors influencing the formation of PAC, identified with simplified mass balance relationships. are the concentration of particles, the concentration of dispersed (non-particle associated) coliform bacteria, and the MCRT. The concentration of dispersed coliform bacteria was found to decline with increasing MCRTs. The rate of decline was greater than the typical half-life attributed to endogenous decay, suggesting that other factors (e.g., predation by protozoa) influence the concentration of dispersed coliform bacteria, and subsequently the formation of PAC. Given that the association of targeted organisms with particles adversely impacts the performance of a disinfection system, studies targeted at the fate of organisms other than coliform bacteria in the activated sludge process are of paramount importance in assessing the health risks of post-disinfected effluents.

Inactivation of Viable Ascaris Eggs by Reagents during Enumeration

ABSTRACT Various reagents commonly used to enumerate viable helminth eggs from wastewater and sludge were evaluated for their potential to inactivate Ascaris eggs under typical laboratory conditions. Two methods were used to enumerate indigenousAscaris eggs from sludge samples. All steps in the methods were the same except that in method I a phase extraction step with acid-alcohol (35% ethanol in 0.1 N H2SO4) and diethyl ether was used whereas in method II the extraction step was avoided by pouring the sample through a 38-μm-mesh stainless steel sieve that retained the eggs. The concentration of eggs and their viability were lower in the samples processed by method I than in the samples processed by method II by an average of 48 and 70%, respectively. A second set of experiments was performed using pure solutions of Ascaris suum eggs to elucidate the effect of the individual reagents and relevant combination of reagents on the eggs. The percentages of viable eggs in samples treated with acid-alcohol alone and in combination with diethyl ether or ethyl acetate were 52, 27, and 4%, respectively, whereas in the rest of the samples the viability was about 80%. Neither the acid nor the diethyl ether alone caused any decrease in egg viability. Thus, the observed inactivation was attributed primarily to the 35% ethanol content of the acid-alcohol solution. Inactivation of the eggs was prevented by limiting the direct exposure to the extraction reagents to 30 min and diluting the residual concentration of acid-alcohol in the sample by a factor of 100 before incubation. Also, the viability of the eggs was maintained if the acid-alcohol solution was replaced with an acetoacetic buffer. None of the reagents used for the flotation step of the sample cleaning procedure (ZnSO4, MgSO4, and NaCl) or during incubation (0.1 N H2SO4 and 0.5% formalin) inactivated the Ascaris eggs under the conditions studied.