Electricity use in 2004 for the water and wastewater industries in the U.S. was approximately 107 billion kWh/yr, or about 3% of retail electricity sales, and is projected to grow to 120 billion kWh/yr by 2010. Currently, energy is consumed in over 161,000 public water systems and over 16,000 publicly owned municipal wastewater treatment works. Energy efficiency improvements and advanced treatment technologies have the potential to slow energy growth while improving water and wastewater processes. The main energy consuming aspects of water systems are distribution pumping, untreated water pumping, and treatment processes, with distribution pumping accounting for the majority of energy use. Though currently relatively small, the share of energy consumption in water treatment processes is growing due to improvements in pumping plant efficiency and requirements for greater levels of water treatment. This paper describes the energy use characteristics of water and wastewater systems and provides a methodology for improving energy management practices.
As part of a project on the use of ultrafiltration (UF) for particle removal, studies were performed to evaluate the use of low‐pressure hollow‐fiber UF as an alternative for complying with Surface Water Treatment Rule (SWTR) requirements for microbial removal and/or inactivation. Pilot studies were conducted on four different untreated source waters, two from northern California and two from Boise, Idaho. Process efficacy was assessed by conducting MS2 virus, total coliform bacteria, and Giardia muris seeding studies, as well as monitoring for naturally occurring bacteria. The study showed that UF was capable of meeting SWTR requirements for alternative filtration technologies without the use of chemical disinfection. Four or more logs of Giardia and more than 6.5 logs of virus were removed from each of the untreated source waters. Differences in water quality or changes in operating parameters did not appear to affect removal capabilities of the process. Maintenance of membrane integrity was critical to assuring process efficacy. When module integrity was compromised, as in fiber breakage, both MS2 virus and G. muris were detected in the permeate. Changes in membrane integrity were not necessarily reflected by changes in permeate turbidity; however, particle counting was an effective method for detecting a compromised membrane module.
As a result of current and anticipated disinfection by‐product (DBP) regulations, increased interest is being shown in pressure‐driven membrane processes for DBP precursor removal. Pilot studies were conducted with two California surface waters and one in Ottawa, Ont. One ultratiltration (UF) membrane with a molecular weight cutoff (MWCO) of 100,000 daltons and four nanofiltration (NF) membranes with MWCOs ranging from 200 to 800 daltons were evaluated. Results indicated that UF was ineffective for controlling the formation of DBPs. When little or no bromide was detected in the permeate, hollow‐fiber NF membranes with MWCOs of 400–800 daltons effectively controlled DBP formation. In waters containing bromide, higher bromoform concentrations (compared with the raw water) were observed after chlorination of the permeate of these membranes. Use of spiral‐wound NF membranes (200–300 daltons) controlled the formation of brominated THMs, but pretreatment of the water was necessary.
Theoretical analysis using a trajectory approach indicated that in the presence of an external electric field, charged waterborne particles are subject to an additional migration velocity that increases their deposition on the surface of collectors (e.g., sand filter). Although researchers conducted bench-scale experiments to verify the effectiveness of electrofiltration, few studies have reported on the applications of electrofiltration in larger scale facilities. In this study, a prototype pilot-scale electrofiltration unit, consisting of an acrylic tank (0.3Â0.3Â1.2 m) with vertically placed stainless steel mesh electrodes embedded in a sand filter was tested at a local drinking water plant. Presedimentation basin water was used as the influent with a turbidity ranging from 12 to 37 NTU. At an approach velocity of 0.84 mm=s, an electrode voltage at 8 and 12 V increased the particle removal coefficient pC* [defined as Àlog(C out =C in )] to 1.79 and 1.86, respectively, compared to 1.48 when there was no electric field. Reducing the approach velocity from 0.84 to 0.42 mm=s increased pC* from 1.48 to 1.64, when the electrode velocity was 16 V. Repetitive experiments were conducted and the results were in agreement with those calculated by a theoretical trajectory analysis. The electrofiltration process was demonstrated to be more effective for removal of smaller particles (<4 mm), the size range of many waterborne bacteria. A voltage of 8-12 V was shown to be the most cost-effective range, considering both the energy cost and filtration performance. The findings from this pilot-scale study are important for full-scale applications of the electrofiltration technology.
It has been customary for water‐treatment plants using chemicals in the water‐treatment process to discharge waste sludge into a nearby natural water course. This practice is coming under increasing criticism from both the public and water‐quality agencies and is now becoming subject to regulation. This article describes pilot work subsequent to the receipt of specific discharge requirements set by the Regional Water Quality Control Bd.
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