Grantee Research Project Results

Final Report: Wastewater Treatment by Pulsed Electric Field Processing

EPA Contract Number: 68D02089
Title: Wastewater Treatment by Pulsed Electric Field Processing
Investigators: Kempkes, Michael A.
Small Business: Diversified Technologies Inc.
EPA Contact: Richards, April
Phase: I
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $99,092
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text |  Recipients Lists
Research Category: Watersheds , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)

Description:

This Phase I research project focused on adapting pulsed electric field (PEF) technology to combined sewer overflows (CSOs). In the PEF process, a liquid is passed through a small treatment chamber, where it is subjected to short (1–10 microseconds) pulses of very high voltage, typically 20–500 kV. The high voltage field created across the liquid (1–35 kV/cm) kills microorganisms by disrupting their cell membranes via an effect called "electroporation," the dilation of the pores in the cell wall. If the electric field is large enough and the duration long enough (microseconds), the pores are irreversibly damaged, and the cell dies.

PEF systems are operated in-line, in a continuous flow process (see Figure 1 for a commercial-scale pulsed power system). To treat liquid flows, the pulses are so short and frequent (hundreds to thousands per second) that all of the liquid in a pipe can be treated as it flows past the electrodes in a treatment chamber.

 

Figure 1. Commercial-scale pulsed power system

By using multiple treatment chambers in series to apply the pulses to a stream of fluid, kill ratios of 5–9 log have been achieved, similar to the effectiveness of pasteurization. Log reduction is shorthand for the ratio of bacteria at the inflow and outflow of a treatment process. A 5-log reduction (typical for food pasteurization) means that 1 bacterium in a sample remains alive for every 100,000 present prior to treatment. Because it uses an electric field, PEF has no chemical impact on the liquid being treated. The energy in PEF processing is sufficient to disrupt living cells, but not powerful enough to create chemical reactions.

During Phase I, Diversified Technologies, Inc. (DTI) addressed the applicability and economics of PEF processing to bacterial reduction on stormwater in CSOs, and its broader applicability to sewage treatment, sludge minimization, and other wastewater treatment applications. During Phase I, DTI worked with U.S. Environmental Protection Agency (EPA) and Massachusetts Water Resources Authority experts in CSO and wastewater treatment to optimize the application of PEF to these critical areas. Phase II of this effort will entail construction and testing of a laboratory/pilot CSO treatment facility, to verify the performance, costs, and benefits of PEF processing of CSO discharges. In Phase III, DTI envisions construction of a full-scale demonstration system, leading to eventual introduction of this technology into CSO facilities across the United States.

Summary/Accomplishments (Outputs/Outcomes):

Defining specific requirements for PEF treatment of CSOs was a significant challenge for two reasons. First, there is no "standard" CSO overflow-in fact, a major aim of sewage treatment facilities and operators is to make CSOs as infrequent as possible. Second, the regulatory environment for wastewater treatment is focused on final outcomes (maximum bacterial densities), not intermediate steps. The majority of PEF research, on the other hand, has been on food processing, where the objectives are relative, in terms of log reductions of surviving bacteria compared to the initial conditions. Bridging this gap between PEF research and EPA requirements has been difficult. The CSO parameters of greatest interest to definition of a PEF system have been bounded, but need to be quantified/experimentally verified in Phase II. The two most significant parameters to PEF processing are fluid conductivity (a measure of the electrical resistance of the flow) and flow rates. The critical results of this task are twofold: (1) PEF can significantly reduce the bacterial load of CSO discharges, and (2) it is possible to tailor this reduction to meet a wide range of potential requirements.

In Phase I, DTI closely examined the potential design of a PEF system for CSO/wastewater treatment applications. The key criterion for this system was the ability to treat large volumes of discharge, without operators, at the lowest capital cost. Using DTI's solid-state pulsed power systems for PEF and other applications as a baseline, data for estimating the cost of the CSO system were derived. DTI further refined this model in Phase I to account for the range of potential bacterial log reductions that may be suitable for CSO treatment.

One issue identified early in Phase I was the need for a new type of treatment chamber. After considering a number of potential solutions, DTI selected a concentric electrode approach, located in a trap (see Figure 2). This approach is similar to the trap in a drainpipe, it ensures that the electrode is immersed, independent of the flow rate.

 

Figure 2. Artist's rendition of trap with concentric electrodes

DTI has a proprietary pricing model for construction of solid-state pulsed power equipment, and independently developed a subset of this model specifically for PEF processing. This model was used to estimate the system cost for a number of CSO treatment scenarios, including the power supply, modulator, and treatment grid, but excluding site preparation, facilities, and the cost of installing the treatment grid in the discharge pipe.

The result of this system design and cost modeling effort is that the capital cost for the PEF equipment, based on low rate production, is estimated at approximately $1–2 million per million gallons per day (MGPD) of peak CSO discharge capability. DTI believes that a 5-10 MW system (capable of treating peak loads of 2-5 MGPD) would be the optimal size for a single system, and multiple systems in parallel could be used for larger discharges. Focusing on a "standard" system configuration, rather than optimizing multiple designs across a number of potential CSO discharge locations, will help drive the system cost lower as production levels increase. Given the adoption of this technology for CSOs and other high volume wastewater treatment applications, this cost is expected to drop by 30–50 percent with the production of 10 or more systems annually.

At these cost levels, PEF is comparable to ultraviolet (UV) or ozonation, without the drawbacks of shadowing due to turbidity and particles, or contact time requirements. PEF is significantly more expensive than chlorination, but does not require substantial contact time for effectiveness, and has no chemical by-products. PEF is, however, substantially less expensive than the cost of CSO elimination, based on several recent and planned projects in Massachusetts.

During Phase I, DTI identified two potential sites and corresponding operating partners for initial PEF treatment demonstrations. In both locations, it is possible to install and operate a small PEF system to collect the required critical data, including:

· Conductivity measurements of combined sewer flows over multiple stormwater events and at varying times during each storm.
· Bacterial characterization of combined sewer flows over multiple stormwater events and at varying times during each storm.

· Validation/verification of the relationships between bacterial reduction at differing PEF parameters measured in food processing systems and those measured in combined sewage flows.

· Development of bacterial dilution estimates based on the ratio of stormwater to sewage in the flow.

Conclusions:

There are several major conclusions, and two significant research and development/demonstration areas identified for Phase II:

· PEF appears to be the only nonchemical process capable of effectively treating CSO discharges. The other identified processes, including UV and ozonation, do not transition effectively from disinfecting clean water to disinfecting CSOs, due to the turbidity and organic content dissolved in the CSO flows. PEF requires very short contact time, is scalable across a wide range of flow rates, and is essentially unaffected by turbidity

· Chlorination is the most common and accepted method for treating CSOs. It has, however, significant drawbacks in CSO treatment. These include minimal contact time; the need to handle, store, and dispense hazardous chemicals accurately (and unattended) into fast-changing flow rates; and the environmental impact of chlorine in the discharge body of water.

· The performance requirements, and site-specific parameters, which would define a PEF system for CSO discharge treatment, have been bounded in Phase I. The characterization of CSO flows and bacterial loads planned for Phase II may have a significant impact on the expected cost of PEF treatment for CSOs at any specific site.

· PEF fills a "middle ground" in terms of cost and impact in dealing with CSOs. PEF treatment is environmentally benign, with no chemical handling or residues. It is, however, significantly more expensive than chlorination. At the same time, it is significantly less expensive than elimination of the CSOs altogether.

· The potential role of PEF in CSO discharge treatment is significant, but not universal. The application of PEF to CSO discharges will be most advantageous where:

• – CSO discharges are a significant and disruptive factor in overall water quality, and must be remediated.
  – Chlorination is impractical, or has significant detrimental impact on water quality (or, as was mentioned repeatedly as a possibility, chlorination is banned or significantly curtailed).
  – The cost of completely eliminating the CSO discharges is very high.

Supplemental Keywords:

pulsed electric field, PEF, combined sewer overflows, CSO, discharge, electroporation, microorganisms, stormwater, wastewater, bacterial reduction, sewage treatment, disinfection, solid-state pulsed power system, chlorination, small business, SBIR., Scientific Discipline, Water, Waste, Wastewater, Municipal, Environmental Chemistry, Civil/Environmental Engineering, Environmental Engineering, wastewater treatment, combined sewage outflows, bacteria, pulsed electric field processing, wastewater treatment plants, municipal wastewater, wastewater systems, stormwater, wastewater discharges, aqueous waste stream