>From the lab to the lawn
In recent years, water problems in Milwaukee, Wis., and Washington, D.C., have commanded headline attention. All the headlines, however, point to a common thread: contaminated drinking water. None of the reported instances of waterborne illness or death has been the result of reclaimed water. Why then, is the public so squeamish about the idea?
Water reuse has become an integral component of water resource management in regions of the United States where traditional water supply sources are being stretched to their limits. Experience at operating water reuse facilities has demonstrated that, from a health standpoint, reclaimed water is safe for a wide variety of uses. Several state regulatory agencies have developed or are developing water reuse criteria that help simplify the planning, design and implementation of projects.
Public health concern over using reclaimed water centers on water quality, treatment reliability and the difficulty of identifying human exposures to potentially toxic chemicals and microorganisms. Eliminating or reducing the concentrations of constituents of concern – through source control, wastewater treatment, and/or by limiting human exposure to the water via design or operational controls – makes reclaimed water safe for reuse applications. While some health-related concerns and issues associated with the use of reclaimed water will require research to resolve, conventional treatment technology and proper control and management techniques can allay most fears.
Issues pertaining to nonpotable reclaimed water applications are more easily resolved than those pertaining to indirect potable reuse, although some questions remain regarding microbial pathogens. The assessment of health risks associated with indirect potable reuse must consider both microbiological and chemical contaminants. It is not definitive because of limited chemical and toxicological data and inherent limitations in analytical methodology.
Although there have been no confirmed cases of infectious disease resulting from the use of properly treated reclaimed water in the United States, the potential for infection is the most common concern associated with nonpotable reuse of treated municipal wastewater. With a few exceptions, the disease organisms of epidemic history are still present in today’s sewage, and the matter is more one of severance of the transmission chain than a total eradication of the disease agent.
The infectious agents that may be present in untreated municipal wastewater can be classified into three broad groups: bacteria, parasites and viruses. In general, viruses are more resistant to environmental stresses than are many of the bacteria, although some viruses persist for only a short time in wastewater.
With a few exceptions, there are minimal health concerns associated with chemical constituents where reclaimed water is not intended to be consumed. Furthermore, while there has been some concern regarding irrigation of food crops with re-claimed water, available data indicate that potentially toxic pollutants do not enter the edible portions of crops.
Chemical constituents are of primary concern in the case of potable reuse. Control needs to be considered where significant quantities of reclaimed water from irrigation or other beneficial uses reaches potable groundwater supplies or where organics may accumulate in the food chain, e.g., in fish-rearing ponds. Industrial wastewater pretreatment programs are effective in reducing the level of many chemical constituents in wastewater.
Water quality criteria alone, particularly those involving surrogate parameters, do not adequately characterize reclaimed water quality. For example, there is little correlation between the concentration of indicator organisms and pathogenic organisms, particularly viruses and parasites, when evaluated in the absence of wastewater treatment processes. Several studies and water reclamation plant operational data indicate that it is necessary to prescribe both treatment unit processes and water quality limits.
Treatment Processes A combination of treatment and quality requirements known to produce reclaimed water of acceptable quality obviates the need to monitor the finished water for health-significant chemical constituents or pathogenic microorganisms. Consequently, expensive and time-consuming monitoring may be eliminated without compromising health protection.
For example, the California Department of Health Services (DHS) has determined that reclaimed water should be essentially free of pathogenic organisms. DHS specifies treatment processes (biological oxidation, filtration, and disinfection), operational requirements (filtration rates, chlorine contact time, etc.) and water quality parameters (turbidity and coliform organisms) that, in combination, have been demonstrated to result in the production of water of the desired quality.
Disease outbreaks caused by Giardia and Cryptosporidium have emerged as major waterborne diseases in non-reuse cases worldwide, but no cases have been linked to water reuse. Still, there are concerns that existing treatment and quality criteria may not adequately address the removal of parasites. Filtration preceded by proper chemical pretreatment, however, can effectively remove parasitic cysts and oocysts, as can membrane processes such as microfiltration.
The Problem of Aerosols While bacteria and viruses have been found in aerosols emitted by spray irrigation systems using untreated and poorly treated wastewater, there have been no documented disease outbreaks resulting from the spraying of disinfected reclaimed water. In fact, studies indicate a low health risk associated with aerosols from spray irrigation sites using disinfected reclaimed water. Limiting exposure to aerosols produced from reclaimed water that is not highly disinfected through design or operational controls ensures the low risk.
Windblown spray of reclaimed water droplets presents a greater potential health hazard than aerosols. Most states with reuse regulations or guidelines include setback distances from spray areas to property lines, buildings or public access areas to prevent direct or indirect human contact with the reclaimed water.
Predictive models have been developed to estimate microorganism concentrations in aerosols or larger water droplets resulting from spray irrigation of wastewater. However, setback distances are somewhat arbitrarily determined by regulatory agencies based on experience and engineering judgment.
Most health experts agree that epidemiological studies of exposed populations at nonpotable water reuse sites are of limited value because of: * the mobility of the population; * the small size of the study population; * the difficulty in determining the actual level of exposure of each individual; * the low illness rate, if any; * insufficient sensitivity of current epidemiological techniques to detect low-level disease transmission; and * other confounding factors. Therefore, most regulatory agencies have chosen not to use such studies as a basis for determining water quality standards.
In contrast, observation of health effects associated with large, stable populations consuming reclaimed water via indirect potable reuse is one way – albeit imperfect – to estimate whether an effect on human health has occurred by long-term ingestion of reclaimed water. However, the minimal observed latency period is about 15 years between first exposure and development of human cancers; therefore, it is unlikely that epidemiological studies will detect cancer incidence and mortality if they are performed before enough time has elapsed between initiation of a potable reuse project and the onset of cancer.
Toxicological tests are performed to evaluate mutagenicity, carcinogenicity and other toxic effects associated with potable reuse. Short-term in vitro tests are poor predictors of potential health effects. In general, long-term in vivo toxicological studies provide a better indication of possible human health effects associated with complex chemical mixtures in reclaimed water.
Risk assessment Because of the insensitivity of epidemiological studies, methodologies have increasingly relied on indirect measures of risk using analytical models. Risk analyses require several assumptions to be made. For example, they must consider: * minimum infective doses of selected pathogens; * concentration of pathogens in reclaimed water; * quantity of reclaimed water or pathogens ingested, inhaled or otherwise contracted by humans; and * probability of infection based on infectivity models. Most microbial risk assessments calculate the probability of individual infection or disease based on a single exposure and do not explicitly acknowledge factors such as secondary spread and immunity. More sophisticated models calculate a distribution of risk over the population by using epidemiological data.
Risk analysis models are not used as the sole basis for regulatory decisions affecting water reuse. Indeed, no reclaimed water standards or guidelines in the United States are based on risk assessment using microorganism infectivity models. However, it is a useful tool in assessing relative health risks associated with water reuse and undoubtedly will play a role in future criteria development as epidemiological-based models are improved and refined.
Indirect potable reuse Indirect potable reuse involves the discharge of treated wastewater into a water course, a raw water reservoir or an underground aquifer. Withdrawal of the water is made downstream or down gradient at a later time for treatment and subsequent distribution as drinking water. Although it may be technically possible to produce reclaimed water of almost any desired quality, some health authorities and others have been reluctant to allow or support the planned augmentation of potable water supplies with reclaimed water.
Using natural waters derived from the most protected source has been the maxim for raw water supplies for almost 150 years. That principle was affirmed in EPA’s 1976 Primary Drinking Water Regulations: “… priority should be given to selection of the purest source. Polluted sources should not be used unless other sources are economically unavailable … .”
Indirect potable reuse, on its face, is less desirable than using a higher quality source water for drinking. Although several studies have indicated that reclaimed water can meet drinking water standards, those standards are not intended to apply to highly contaminated source waters such as municipal wastewater; thus the product water may have to meet additional water quality criteria.
Quality standards have been established for many inorganic constituents, and treatment and analytical technology has made it possible to identify, quantify and control those substances. Similarly, technology can eliminate pathogenic agents from contaminated waters.
On the basis of available information, there is no indication that health risks from using highly treated, reclaimed water for potable purposes are greater than those from using existing water supplies.
However, unanswered questions remain with regard to organic constituents, due mainly to their potentially large numbers and unresolved health risks resulting from long-term exposure to extremely low concentrations. A multiple barrier system using demonstrated treatment technologies is essential to assure the public that reclaimed water used to augment drinking water supplies is as safe and reliable as other alternative supplies.
Guidelines and Criteria Reclaimed water quality standards, particularly microbiological limits, vary considerably among states. However, states with extensive reuse experience have comparable, conservatively based criteria or guidelines.
Arguments for less restrictive standards are most often predicated on a lack of documented health hazards rather than upon any certainty that hazards are small or nonexistent. In the absence of a common interpretation of scientific and technical data, selection of water quality limits will continue to be somewhat subjective and inconsistent among the states.
Public Opinion According to a recent survey conducted for the San Francisco Department of Public Works, Fire Department and Water Department, San Franciscans strongly support water recycling.
“They support conservation,” says Karen Qubick, Project Manager for the Public Utilities Commission. “They believe the city should make the most of its resources. Recycled water is pretty commonplace in California, and residents are very proud to be doing their part.”
San Franciscans further support the city’s effort to use recycled water through the Supplemental Water Supply Program. The program is intended to expand the Auxiliary Water Supply System (AWSS). AWSS currently uses reclaimed water for dust control, soil compaction and street cleaning. Future plans for the project will include fire-fighting capabilities, flushing toilets in commercial buildings, and irrigating parks and golf courses.
According to Qubick, residents demonstrate their support for water recycling by their willingness to pay for the program. In the survey, a majority of those who support water recycling said they would be willing to pay between $2 and $3 each month to help cover the cost of the program.
The city government also encourages the use of reclaimed water through its laws. “Currently, we have what we believe to be a unique ordinance that states that new buildings must be ready to use reclaimed water when we have it,” Qubick says.
The greatest potential reclaimed water demand in San Francisco is for landscape irrigation (8.7 million gallons a day), and the primary customer for this water is the city’s Recreation and Parks Department. It undertook a reclaimed water irrigation pilot study at Golden Gate Park, which involved watering 26 species of container plants and five species of bedding materials, all selected by the nursery gardeners.
Each experiment site had two identical sets of plants, one irrigated with recycled water and the other with control water. Park gardeners carried out routine care and maintenance. Water, soil and plant tissue samples were then taken by the department of public works and studied.
“At the end of the two-year study, there was little or no difference between plants watered with reclaimed water and the plants watered with new water,” Qubick says. “This was a showcase project. People can come and see the experiment and note for themselves the results and tell that there really is no difference between new and used water.”
In St. Petersburg, Fla., reclaimed water has also been used successfully.
“We have the largest water reclamation facilities in the world,” says St. Petersburg director of utilities, Bill Johnson, “We cater to 9,000 homes and businesses. We have exhausted our water supply, and we can’t store it, so recycling it was the best solution.”
St. Petersburg’s 300 mile water system connects four water reclamation facilities. The system serves golf courses, schools, parks and cooling towers, but more than 95 percent of the connections are for residential irrigation.
“The typical residential lawn can require up to 30,000 gallons of irrigation water per month during the growing season,” Johnson says. “But the average sewage customer only discharges 6,000 gallons per month to the system. It takes five sewer customers to produce enough reclaimed water to supply one home with irrigation water. As a result, it is not presently possible to supply all the homes in St. Petersburg with reclaimed water.”
St. Petersburg’s recycling system is vital in helping the city meet its long-term water needs. Johnson estimates that, by 2000, the city’s reclaimed water system will be able to serve approximately 17,000 customers and irrigate almost 9,000 acres.
Johnson says several steps need to be taken to ensure the success of wastewater reuse programs. First, the recycled water system should be readily identifiable. St. Petersburg color-codes all PVC piping, using blue for potable water, green for sewer force mains and brown for reclaimed water.
All buried iron piping has brown tape to denote it as part of the reclaimed water system. Fire hydrants also are color-coded throughout the system with brown stems and yellow caps.
“Hydrants are needed to periodically flush lines and serve as backup for fire suppression,” Johnson says. “Valve box covers for reclaimed water are shaped differently than potable water system valve box covers. Also, all customers using reclaimed water must have an in-ground irrigation system. Hose bibs are not permitted at any point in the irrigation system.”
In addition, wherever reclaimed water is available, backflow prevention devices have been installed at every potable water system meter. And finally, the city has adopted comprehensive rules and regulations governing connection to and usage of the system.
“If wastewater is considered a valuable resource requiring proper treatment and handling by qualified technicians at all times, the public will accept reclaimed water with a high degree of confidence,” Johnson says.
James Crook is director of water reuse in the Boston office of Kansas City, Mo.-based Black & Veatch.
The Water Environment Research Foundation has published a report containing the findings of a study of efficiency in wastewater operations nationwide. The report, Benchmarking Wastewater Operations: Collection, Treatment and Biosolids Management, is based on a survey of more than 100 wastewater utilities and identifies the practices that lead to efficient operation of wastewater treatment facilities.
The report points to the Houston Department of Public Works and Engineering’s Public Utilities Group as one of the nation’s most efficient wastewater utilities, citing its ability to reduce costs while improving performance. In the last fiscal year alone, the group has reduced its costs by $15 million.
The report explains benchmarking efforts specifically focused on wastewater collection and treatment and management issues. Additionally, it identifies characteristics common to well-run wastewater utilities. For example, leading utilities had many practices in common, including: * sharing their workforce with other utilities or other parts of their organization; * avoiding unnecessary costs by influencing regulatory outcomes with good science; * using efficient communications technology and work order generation; * managing competition to challenge the work force, and outsourcing for reduced costs when applicable; and * providing extensive training and cross-training.
The report (Order No. D73001PL) is available from the Water Environment Federation. WERF subscribers may call (703) 684-2470. Water Environment Federation members and others may call (800) 666-0206 or (703) 683-2452. The price is $10 for WERF subscribers, $55 for WEF members and $75 for nonmembers, plus postage, handling and applicable taxes.
Junction City, Kan., officials have turned a wastewater treatment facility and a water treatment plant from detriments to assets and, in the process, gained a powerful economic development tool. The two treatment systems had a blemished history. Odor and environmental compliance problems, along with poor management skills, a lack of technical expertise and flawed budgeting practices, convinced the city to seek help.
In 1989, Junction City officials decided to outsource the operation, maintenance and management of the plants. In the eight years since, the city and Houston-based Professional Services Group have become partners not only in plant operation, but in planning for the city’s growth, says City Engineer Tom Neal.
Improvements to one of the city’s two wastewater treatment plants helped increase the efficiency and reduce the cost of the plant’s biosolids processing operation by nearly 50 percent. Changes included the replacement of worn-out equipment, painting of all facilities, expanding and centralizing all permit-required testing, converting an unused area into an employee lunchroom, adding air conditioning in the plant’s main offices and upgrading pump stations for higher capacities.
The contract stipulates that municipal funds, (e.g., for chemicals, repair, maintenance and sludge disposal) left unused at the end of a fiscal year are returned to the city and held in a payback reserve. The proprietary funds are used to assist the city in paying for large-scale capital improvement projects associated with the water and wastewater treatment systems. In the last eight years, approximately $205,000 has been returned to the fund.
The city receives monthly reports, including a financial report and a maintenance record report, on the systems. The operating company also handles all discharge report filings.
The new 2.5 mgd wastewater treatment facility was designed by Metcalf & Eddy, Wakefield, Mass., to treat both industrial and residential wastewater. The project was completed in 39 weeks for $6.5 million and was paid for through a bond issuance. The groundbreaking was held in February 1996, and, by November 1996, the plant was receiving wastewater flow
The Lower Salford Township Authority, Pa., was experiencing regular problems with the progressive cavity pumps that provided service from the primary clarifier in its return activated sludge (RAS) application at their wastewater treatment plant. Grit on the primary side caused heavy wear on both the rotors and stators, and a major overhaul, costing $12,000 for parts alone, had to be performed every two years. Bearings and seals needed to be replaced every three to six months, a process that cost an additional $1,500.
In an effort to solve the problems at the city’s wastewater treatment plant, the authority decided to replace the old pumps with rotary lobe pumps from Hayward Gordon, Buffalo, N.Y. The new pumps operate by gently drawing the product through the inlet port and into the pockets formed between the rotors and the housing.
The pockets of fluid rotate with the rotors 180 degrees around the housing until they are discharged through the outlet port. The pump design minimizes contact between the pump inner surface and abrasive or corrosive media.
Incoming raw sewage, with a BOD averaging 225-250 mg/L and total suspended solids of 175 mg/L, is pumped into two 21,000-gallon tanks and two 14,000-gallon RAS tanks. The pumps then return sludge (150 gallons per minute) back into the system where it is re-aerated and mixed with incoming sewage. Fine air diffusers in the contact tank improve the oxygen transfer rate.
The pump design consists of two dual or tri-lobe rotors driven and supported in the pump casing by a heavy-duty timing gear/bearing housing. Suction and discharge connections are rectangular flanges that allow for a variety of bolt-on connection flange sizes and types. The pump is symmetrical about the vertical centerline and can operate in either direction with installation of a reversing starter.
The primary RAS pumps include: * a reversible, hardened front cover wearplate; * replaceable, hardened rear housing wearplates; * laser-hardened center housing internal wear surfaces; * replaceable center housing wearplates; * replaceable Buna rotors; and * hard-surfaced, mechanical seals. Lower Salford’s pumps have been in service for nearly two years, and routine inspection shows no signs of wear on the lobes or interior surfaces.
This article was written by Lewis Christy, plant manager for Lower Salford Township Authority, Harleysville, Pa.
Lynchburg, Va., has restored an interceptor that was so severely blocked by debris that every rainstorm, even a moderate 10-minute shower, would surcharge it. Excess flow threw manhole lids aside, flooded streets and basements, and caused raw sewage to bypass into Blackwater Creek and the James River.
Extraneous materials (sludge, rocks, grease, old clothing, tree trunks, corrugated metal and other solids and semisolids) had filled more than 80 percent of the 18-inch to 60-inch lines. Additionally, much of the interceptor had been constructed on a sideslope, through heavy woods, along a narrow right-of-way, making it difficult to access.
Lynchburg officials eventually awarded the cleanup project to Robinson Pipe Cleaning Co., Eighty-four, Pa. Cleaning cost the city only $1.3 million; installation of a new line could have run as high as $8 million.
The contractor used a vacuum jetter to clear the 1.21 miles of line at the upstream end of the James River interceptor by applying 120 gpm of hydrant-supplied water at 3,000 psi, and vacuuming up the resulting slurry. Crews then tackled the main parts of the project: the Blackwater Creek interceptor (1.08 miles) and the James River interceptor (5 miles).
Pipes ranged in size from 32 inches to 60 inches, and some were big enough to allow personnel to enter. However, the entire job was handled from ground level so there was no exposure to possible gas accumulation or collapsing pipes.
Bucketing the pipes proved much faster and more effective than hydraulic cleanout methods alone. Repeated winching of pipeline buckets, sized several inches smaller than the pipe itself, took out most of the debris. One to two jetting passes at 125 gpm and 2,500 psi scoured the pipes to 98 percent cleanliness.
Crews worked through three manhole lengths at a time. The solids were hoisted from the center manhole by either hydraulic clamshell or vacuum loading and hauled in dump trucks for disposal. An average of 18 to 20 tons of material was removed per 10-hour work day.
A closed-circuit television camera took care of the inspection phase of the project. The cables pulled through and cleaned piping at speeds up to 30 feet per minute, while a wide-angle lens captured views of the entire diameter of the pipe.
Camera movement could be adjusted so problem areas could be image-printed. Manhole inflow was documented, along with other points of significance, such as root intrusion, defective connections, broken pipe and the presence of scale and corrosion.
When the project was completed, 10 months after it had begun, nearly 3,764 tons of debris had been removed. The city’s water/wastewater interceptor had been restored to its full design capacity without any interruption of service.