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Filter backwash recycling: A safe water resolution
Sounding like a software customer who has bought the latest version of his favorite program, the U.S. Environmental Protection Agency (EPA) is telling America’s water systems to get the bugs out.
8 Min Read
Written by Mark Waer and Leland Harms
Sounding like a software customer who has bought the latest version of his favorite program, the U.S. Environmental Protection Agency (EPA) is telling America’s water systems to get the bugs out. The Filter Backwash Recycling Rule (FBRR), which goes into effect this year, is designed to reduce the likelihood that pathogenic organisms — primarily Cryptosporidium oocysts — will pass through the treatment process into finished drinking water. Water purveyors support the rule’s public health goal, and treatment plant operators must now determine how to comply with provisions of the rule while minimizing additional treatment costs.
Preserving health and water
The 1996 Amendments to the Safe Drinking Water Act required EPA to regulate the recycling of filter backwash water within the treatment works of public water systems. The requirement was prompted by the concern that water treatment plants might be concentrating pathogenic microorganisms in solids removal processes such as sedimentation, then recycling the concentrated waste streams back into the treatment process.
When concentrated waste streams are added to untreated raw water containing an initial concentration of pathogens, the overall concentration of pathogens will gradually increase until final equilibrium is reached. The danger is that a plant’s pathogenic removal processes will not be adequate to protect the public, and some dangerous organisms will be released into the treated water.
Many water treatment plants, especially those using conventional treatment or direct filtration, recycle waste streams to minimize overall water loss. Recycling those waste streams is especially important in arid areas such as the Southwest.
In the greater Phoenix area, for example, most of the municipal water treatment plants recycle one or more of their waste streams. Don Gage, plant operations supervisor for the Cholla Water Treatment Plant in Glendale, Ariz., estimates that recycling backwash water at his plant conserves approximately 100 million gallons of water annually.
As a rule …
The FBRR, published in final form in the Federal Register on June 8, 2001, will take effect at the local level on Dec. 8, 2003. Most utilities will achieve compliance through their appropriate state agencies and will not report directly to EPA.
The FBRR includes provisions for recycling filter backwash water, sludge thickener supernatant and liquids from solids dewatering processes. The regulations apply to public water systems — regardless of size — that (1) use surface water or groundwater directly influenced by surface water, (2) recycle one or more of the specified waste streams, and (3) use direct or conventional filtration in treatment processes. (Systems that use other filtration systems — e.g., membranes and diatomaceous earth filtration — or that do not recycle backwash are not affected by the rule.)
The FBRR requires treatment plants to return all recycle streams to the treatment process prior to coagulation, unless otherwise approved by the state. To comply with that provision, most plants will need to return the recycled flow stream to the head of the treatment plant.
In addition to setting process requirements, the rule directs water systems to file their recycling plans with their states. Each system must provide:
a treatment plant schematic showing unit processes, points of chemical addition, and the entry point of all recycle streams, and
flow data in gallons per minute, including the typical recycle flow, maximum plant flow during the previous year, design flow, and, if available, state-approved operating capacity. Many facilities may need to install additional flow measuring devices to comply with that portion of the rule.
Water systems affected by FBRR also will have to maintain records regarding:
information submitted to the state,
all recycle flows and the frequency of recycling,
average and maximum backwash flow rates through the filters,
the average and maximum duration of the backwash process,
typical filter run lengths with a written summary of how the length is determined,
the type of treatment provided for the recycle flow, and
recycle treatment units (i.e., physical dimensions, hydraulic loading rates, type and dose of chemicals used, and frequency of solids removal).
Because the FBRR falls in the “treatment technique” category of regulations that require public notice, water systems that violate the rule have 30 days to notify the public of failure to comply. Systems will be allowed to petition the state for approval of alternate recycling locations provided that they can show at least a 2-log or higher removal when recycling to the alternate location.
The major costs of implementing the FBRR will come from relocating the return flow to allow the recycled waste streams to receive full treatment. EPA estimates that fewer than 400 U.S. water systems will be required to make system modifications to comply with that portion of the rule.
The agency estimates that capital costs associated with modifications will total approximately $45 million. Local governments also will incur administrative costs for gathering, reporting and reviewing data.
According to EPA, costs for FBRR compliance can be recovered in a water rate increase of $0.19 per year for the average household. Of the 31 million households affected by the rule, 99 percent will pay less than $1.70 per year in additional fees; the highest cost would be approximately $100 per year.
A modified outlook
Although the deadline for FBRR compliance is almost a year away, many local governments have begun modifying their treatment plants to meet the rule’s provisions. Among those is Flagstaff, Ariz., which is making capital improvements to its Lake Mary Water Treatment Plant.
Built in 1966, the Lake Mary plant treats up to 8 million gallons of water per day. Processes include coagulation, flocculation, sedimentation, filtration and chlorine disinfection. In addition to the process facilities, the plant includes raw water and finished water pumps, chemical storage and handling facilities, and sludge drying beds.
The plant was designed to operate as two parallel trains treating 4 million gallons of water per day. The existing filters — three in each train — are submerged. Water flows through the sedimentation basin area into the filter area, which is an extension of the common basin and contained within the same walls. The water passes through the filters, into the clearwell.
Because the flocculation and sedimentation basins are connected directly to the filters, operations staff must take half of the treatment plant off line to backwash the filters. Backwash water flows over the filter wall into the sedimentation basin, and, after an appropriate settling time, the treatment train is placed back in service. The arrangement fails to meet FBRR requirements because the backwash water is placed directly in the sedimentation basins downstream of coagulant addition and flocculation.
As a result, Flagstaff is implementing multiple changes. For example, the city is:
abandoning the existing filters, filling them with granular material and topping them with concrete;
constructing weirs and effluent collectors above the existing filters on columns placed on existing filter walls;
constructing a new building to house four dual-media filters with air backwash and filter-to-waste capability;
constructing a recovery pond for backwash and filter-to-waste streams (an existing pond stores sludge from the clarifier); and
constructing a vault, where decant will be recycled and where the backwash and filter-to-waste streams will be recycled prior to the addition of coagulant.
The modifications are currently under construction, and the city expects to have the upgraded plant in service this May. The budget, including improvements to the plant’s chlorine system and additional ultraviolet disinfection, is approximately $4.5 million.
Although the FBRR affects relatively few water utilities, it will produce substantial health benefits and preserve valuable water resources in water-short areas. Communities concerned about funding yet another water-related mandate can take heart in the fact that costs for compliance will be, in most cases, minimal. EPA estimates that necessary plant modifications can be made for an average annual cost that is less than the cost of one can of soda per household.
Mark Waer is a senior water treatment technologist, and Leland Harms is a senior process engineer, for Kansas City, Mo.-based Black & Veatch.
For a copy of the Filter Backwash Recycling Rule, visit the EPA Web site at www.epa.gov.
Rule targets Crypto and other pathogens
The principal benefit of the Filter Backwash Recycling Rule (FBRR) is the control of Cryptosporidium and other disinfection-resistant microbial pathogens. Cryptosporidium is a protozoan parasite that has been isolated from many surface water sources. It is present in the environment in a dormant form known as an oocyst.
When entering the digestive tract of a host organism, such as a human, the oocyst emits four sporozoites through a split that forms in the oocyst wall. The sporozoites then attack the gut, preferably the ileum of the small intestine and reproduce, causing cryptosporidiosis. The infectious dose — the number of organisms necessary to cause a human infection — may be as low as one oocyst.
Cryptosporidiosis normally lasts approximately two weeks, and it cannot be treated with antibiotics. The symptoms, which include watery diarrhea, stomach cramps and nausea, can dramatically dehydrate small children, elderly people and pregnant women. The illness can be life-threatening for people with compromised immune systems.
Several outbreaks of cryptosporidiosis have been attributed to drinking water sources. Most notably, Milwaukee had an outbreak in 1992, in which an estimated 400,000 people were infected and approximately 50 people died.
Cryptosporidium oocysts can persist in the environment from a few weeks to a year, depending on conditions. Because they are extremely resistant to disinfection with chlorine and chloramines, viable oocysts tend to be concentrated in the waste streams of treatment plants. Implementation of the FBRR is intended to reduce those concentrations and the likelihood of waterborne outbreaks.
— Waer and Harms
