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Graywater and Graywater Disposal Systems

There’s been lots of talk recently that wastewater disposal on Cape Cod should evolve toward the use of composting toilets with innovative graywater disposal systems. Graywater is often seen as a benign material which poses a minimal threat to human health and the environment. For this reason, graywater has often been considered a good candidate for various types of reuse or disposal options rather than disposal in a conventional septic system. In this chapter we look at several alternative designs which have been proposed locally for disposal of graywater. We consider the environmental impacts of graywater, and try to balance the cost of alternative disposal options against environmental gains they may provide. We also look at the public health considerations which must be incorporated into any graywater disposal design.

What is Graywater?

Figure 1. Relative Contributions of various sources to household wastewater.

Graywater includes all household wastewater that doesn’t come from toilets. This includes wastewater flows from baths/showers, clothes washing, dishwashers and, optionally, kitchen sinks. Toilet wastewater, and often garbage disposal waste, is called blackwater. Graywater comprises about 60% of typical household wastewater flow (Figure 1).Separation of graywater and blackwater is achieved through the use of separate plumbing. Wastewater separation is relatively easy to accomplish in new construction, but can range widely in cost and ease for retrofits of existing dwellings.

Bacterial Pathogens and Indicators

It is important to recognize that graywater is not always pathogen-free. Under some circumstances graywater may contain more pathogens than are normally found in combined (gray and black) wastewater. This is particularly true in households where a resident is sick or with infants where diapers are routinely laundered.

Numerous pathogenic organisms and microbial indicators have been found in graywater. These include coliform bacteria, fecal coliform including E. coli, (including enterotoxigenic strains), fecal streptococci including enterococci, Salmonella spp., Shigella spp., Vibrio cholerae, Campylobacter jejuni, Yersinia enterocolitica, Aeromonas hydrophila, Legionella pneumophila and other Legionellaceae, Mycobacterium tuberculosis and other mycobacteria, Staphylococcus aureus and other staphylococci, Pseudomonas aeruginosa, Klebsiella pneumoniae, Clostridium perfringens, enteric viruses including polioviruses, coxsackieviruses, echoviruses, hepatitis virus A, Norwalk virus, caliciviruses, astroviruses, reoviruses, rotaviruses and adenoviruses, and possibly Giardia cysts and Cryptosporidium oocysts. Many of these organisms are opportunistic pathogens which are more likely to affect older people, young children and individuals who are immuno-compromised (i.e. individuals who are HIV-infected, chronically ill, or are undergoing chemotherapy). Some of these organisms, such as enteric bacteria and viruses and Staphylococcus, are commonly found on the skin or in the oral cavity of humans and are shed routinely during bathing. According to Management of Small Waste Flows produced by the Small Scale Waste Management Project at the University of Wisconsin-Madison (USEPA publication EPA-600/2-78-173, 1978) bacterial indicators are higher in bathing wastewater than in laundry wastewater (Table 1) due to shedding. In comparison, mean concentrations of fecal coliform and fecal streptococci in septic tank effluents are only 10-100x greater than the high values cited for bathing wastewater. In recognition of the public health risks, conventional septic systems are designed to ensure that there is no surface breakout of septic tank effluent. Similarly, surface discharge of graywater, which may contain similar microorganisms at only 1-2 orders of magnitude lower concentration than septic tank effluent, should be controlled. Graywater should not be applied to the soil surface in areas where humans or pets have easy access unless the graywater is first disinfected.

Source Fecal Coliform per 100ml Total Nitrogen mg/l Toilet
Toilet 1,000,000 200
Bathing 1,000 20
Kitchen Sink 75
Laundry 100 20
Garbage Disposal 80

Table 1. Fecal coliform and nitrogen in wastewater

Source BOD (5-Day) TSS Total Nitrogen Total Phospohrus
Toilet 6.9 – 24 13 – 37 4.1 – 17 0.6 – 1.6
Garbage Disposal 11 – 31 16 – 44 0.2 – 0.9 0.1
Graywater 25 – 39 11 – 23 1.1 – 2.0 2.2 – 3.4

Table 2. Wastewater Pollutant Contributions in grams per person per day.

Other Pollutants

In general graywater is perceived as being relatively clean and low in nutrients or other pollutants. While it is true that graywater is relatively low in nitrogen (most of which is found in toilet wastes), graywater can contain significant amounts of BOD, phosphorus and total suspended solids (Table 2). Biochemical oxygen demand (BOD) in graywater may equal or exceed the BOD content of blackwater. This is primarily because of food waste and grease from kitchen drains. Also due to kitchen sink flows, total suspended solids (TSS) may equal those found in toilet wastes and will far exceed toilet flows when a garbage disposal is used. The phosphorus content of graywater may equal that found in toilet wastes due to the continued use of phosphate-containing detergents. Massachusetts prohibited the sale of phosphate-containing detergents in July 1994, with the exception of detergents used for automatic dishwashers. In a typical residential home these detergents can contribute an amount of phosphorus equal to that found in toilet wastes.

General Considerations for Design of Graywater Systems

Graywater must be treated before discharge. The conventional treatment method, a septic tank large enough to provide at least a two-day retention time, allows grease to cool, solidify and float to the top. This is particularly important when kitchen wastes are part of the graywater flow. Also, longer retention times allow fine solid materials suspended in the wastewater to sink to the bottom of the tank.

The 1995 Title 5 allows the use of a filter in place of a septic tank when only graywater is being disposed of. Several proprietary filters are available. For example, Clivus New England Inc. markets a three-stage aerobic intermittent sand filter. Graywater first flows through a mesh filter which catches large solids and then through a layer of coarse-grained sand. It next passes through a screen filter and into a proprietary media where biological oxidation occurs. Biomat is expected to form at each filter stage to attenuate bacteria and trap solids. There is limited information on the effectiveness, longevity and maintenance requirements of this and many residential graywater filters. In designing or evaluating any graywater filter it is important to recognize that the grease and kitchen solids found in graywater may clog filters. The BOD and nutrients present will promote bacterial growth which may also clog filters and necessitate frequent cleaning.

Increased longevity of the leaching system and reduced maintenance of the gray water system may be achieved if the kitchen sink is not plumbed into the graywater system. Garbage disposals should be strongly discouraged as they can stress even a conventional septic system and will provide an overwhelmingly high solids load to a system designed to handle only graywater. If a garbage disposal is installed it should be plumbed to the blackwater system.

That Innovative Designs are Available for Disposal of Graywater?

Reduced Size Conventional Leaching Facility

The simplest way to dispose of graywater is a conventional soil absorption facility. Under the 1995 Title 5, if graywater alone is discharged the leaching facility may be sized for 60% of the building’s design flow. A septic tank is not required for disposal of graywater only. A filter system specifically approved by DEP may be used in place of the septic tank as long as no garbage disposal waste or liquid waste from a composting toilet enters the graywater disposal system. A conventional leaching facility will generally be the lowest cost alternative for disposal of graywater, averaging $2000-3000 to install. How thoroughly does a conventional leaching field treat graywater? Properly sited leaching facilities with adequate separation to groundwater will act efficiently to remove bacteria and many viruses through simple filtration and adsorption. Filtration will also provide almost complete removal of total suspended solids. Substantial reductions in BOD should also be achieved as the effluent passes through the biomat which surrounds the leach field. The amount of phosphorus removal will largely depend on the soil type in which the leaching system is installed: sands generally have less capacity to attenuate phosphorus while clay or silt soils may attenuate significant amounts of phosphorus. Conventional leaching facilities also provide recharge to the groundwater which other innovative designs may not.

Shallow Drip Soil Absorption Systems

A relatively new design for disposal of graywater is the shallow drip soil absorption system. This system has been used in agriculture for many years to irrigate crops using a network of shallow underground pipes fed by a pump. It delivers water at a controlled rate for uptake in the root zone minimizing percolation of water. When used to dispose of graywater the drip system also has the advantage of using the treatment capability of the surrounding soil to further treat the graywater. Nutrient and organic constituents in the effluent are removed by vegetation or are degraded by microorganisms as effluent moves through the soil. Thus, the quality of effluent treatment is directly linked to the soil and site characteristics such as soil permeability, drainage, slope, and depth to limiting conditions such as bedrock or groundwater.

How it works: graywater first enters the pre-treatment unit (a septic tank, sand filter, etc.) and flows by gravity to a pump chamber or dosing tank. The collected effluent is periodically pumped under pressure to the subsurface drip field. The drip field consists of parallel rows of polyethylene tubing with drip emitter holes (pinhole size) at about two-foot intervals. The emitters distribute the effluent at a slow and controlled rate to a large surface area of soil. This allows the system to operate over long periods without saturating the surrounding soil. It also allows the system to be installed at a shallow depth, usually 6 to 18 inches below the surface. An additional benefit of the design is the use of treated effluent to water shrubbery and gardens.

The drip emitter design is often modified in northern climates where the ground freezes solidly in winter. In addition to the drip tubing these designs incorporate a deeper leaching trench of pressure-dosed perforated PVC pipe laid in a gravel bed 24-36 inches below the ground surface. Piping from the dosing tank leads to a series of valves so that graywater flow can be shunted to either the drip irrigation bed or the deeper leaching trench depending on season of the year.

A potential problem with drip emitter systems is biological and chemical clogging of filters, drip lines, and emitters. Chemical clogging can be caused by a high solids content in the effluent. Residual solids may deposit in the emitter holes if water evaporates out of it between doses, and the resulting buildup of solids around the drip hole may eventually slow the system’s rate of flow. Similarly, salts deposited by evaporation of effluent may also form deposits around the emitter holes and slow flow. Clogging can generally be avoided if the system is flushed routinely (2-4 times per year). This can be accomplished by designing the system so that a garden hose can be connected and high pressure water forced through the system. Alternatively, use of a sand filter to pretreat the effluent before it flows to the drip tubing may eliminate the need for flushing. The sand filter greatly reduces the total suspended solids and BOD content of the effluent which should minimize biological clogging of the system.

A typical household drip disposal system costs about $4000-6000 to install. While this is more expensive than a conventional disposal system it can compare favorably with other alternative disposal options, especially where there are limiting site conditions such as slowly permeable soils or high groundwater. An additional cost of operating the system is the cost of hiring a certified operator who will oversee and periodically flush the system.

Where may the installation of shallow drip disposal systems be most appropriate? The goal of a drip system is to make maximum use of soil treatment capabilities. Drip disposal systems may work well in sandy soils where the effluent is applied in the finer textured topsoil layer where it receives better filtration than it would otherwise receive in the underlying sand. Also, effluent filters through the soil more slowly than in a conventional system, providing better attenuation of pollutants in soils with a fast percolation rate. Because of the slow rate at which effluent is applied, drip systems may also be suitable for marginally usable clay or silt soils where conventional systems will not work.

Other Shallow Disposal Systems

Another type of shallow disposal system utilizes 1 inch perforated PVC pipe laid at 6-18 inches below the surface. The system design is similar to the drip system, including a pretreatment unit and dosing chamber which pressure doses the disposal field. The irrigation bed can be laid at a depth of 6 inches directly in the topsoil, or at a depth of 18 inches in the underlying soil (with or without gravel surrounding the pipe). Many systems use pipe installed at both 6 and 18 inches for summer and winter use, respectively.

As with the drip system, treatment of the effluent is accomplished by biological processes in the surrounding soil and by the fact that effluent is applied at a slow rate. Thus, installation of this type of system is appropriate in situations similar to those discussed for drip systems. The system may also be better able to handle solids than a drip system because the holes in the PVC pipe are larger than those in the drip tubing and are thus less likely to clog.

Closed Evapotranspiration Systems

Yet another strategy for disposal of graywater is to drain the filtered graywater to a specially constructed sealed garden unit. In this type of system graywater is distributed either by pressure or gravity through a series of pipes laid in a gravel bed within a lined excavation. A layer of sand is laid over the gravel and planted with selected plants. The sand acts as a wick to draw the water to the surface for evaporation while the plants take up the graywater by their roots, utilize the nutrients for growth, and transpire the water as vapor. Designs of this type are constructed as no-discharge units where theoretically all water is disposed of by evapotranspiration. The units may be located either indoors in a greenhouse-type design or outdoors in constructed beds.

Several design considerations are key to the success of these units. Firstly, success of the system depends on the evaporation rate of the bed exceeding the rate of effluent loading plus precipitation. This may necessitate a very low effluent loading rate, especially for beds constructed outdoors. Secondly, the sand or gravel used to construct the bed must be large grained enough to ensure that the bed drains thoroughly and remains aerobic. If more water is received than can be evaporated, or if the bed drains poorly, the soil pore space will remain saturated and will tend to go anaerobic causing the plants to die. In addition, if the bed drains poorly there is the chance that graywater will come to the soil surface which is not desirable (and also not legal under Massachusetts health regulations). Evaporative capacity of the bed will vary with both climate (solar radiation, temperature, humidity, wind speed, precipitation) and with the type of plants selected for use. If the bed is constructed outdoors it will likely function at reduced or minimal capacity in winter when precipitation exceeds evaporation. In this case, an alternative provision such as leaching trenches or a tight tank should be made for graywater disposal.

A potential long-term problem with evapotranspiration systems is the buildup of salts left in the soil by the effluent as it evaporates. With time salt concentration may increase to the point where it is harmful to vegetation. Plants in the beds may also have to be fertilized occasionally since graywater usually contains little nitrogen.

Figure 2. Schemata of Graywater Evapotranspiration Bed Installed in Wellfleet

Cost of installation can vary widely largely based on whether the unit is constructed indoors or out. The costs of household units consisting of a lined bed constructed outdoors can vary widely. One recently installed in Wellfleet cost approximately $ 5,000 (Figure 2). If a tight tank must be installed to handle excess flow this will present an additional cost to install and pump. Because evapotranspiration systems produce zero discharge (and are usually coupled with a composting toilet which should also create zero discharge), these systems are most appropriate where soil conditions are limiting (high groundwater, impermeable soils) or where inadequate setbacks to wells or watercourses exist.

It should be recognized that evapotranspiration units are somewhat experimental and often require some trial and error with dosing rate or plant types used before they function properly. Because there is no standard design for evapotranspiration units all designs should be viewed with scrutiny before approval. These designs must also be approved on a case-by-case basis by DEP before installation. A detailed design module entitled Onsite Wastewater Disposal Evapotranspiration and Evapotranspiration Absorption Systems produced by National Small Flows Clearinghouse is available by request to our department for Boards of Health or engineers who may be considering designs for systems of this type.

Scenarios where Innovative Graywater Systems may be Proposed

What are the most likely scenarios that Boards of Health will encounter where innovative graywater disposal systems are proposed? Where may innovative graywater designs provide a good solution to wastewater disposal?

Where an existing septic system is failing hydraulically, it may be possible to install composting toilets and allow graywater to flow to the existing cesspool or septic system. In repair situations (remedial use), DEP will allow graywater to be disposed of in an existing cesspool under the following conditions: a composting toilet is used for human waste; there is no discharge of garbage grinder waste or liquid waste from the composter; the cesspool is pumped and cleaned; the cesspool is not located in groundwater; the cesspool meets the design criteria of 310 CMR 15.253 (design criteria for pits, galleries, or chambers); and the effluent loading requirements of Title 5 are met. Because the total volume of water is reduced and because graywater contributes a lower solids and BOD load than combined wastewater the existing septic system may be able to hydraulically function if it receives only graywater. A number of systems of this type have been approved by DEP elsewhere in Massachusetts and seem to be functioning well. This appears to be a cost effective way for homeowners to deal with some hydraulically failing systems.

Where an existing septic system has been deemed to have failed because of insufficient setbacks to wetlands or watercourses, use of the existing septic system or an innovative shallow trench design for graywater only may also be appropriate. Setbacks to watercourses are determined primarily to ensure that pathogens are removed by soil filtration before wastewater intercepts a watercourse. Removal of the toilet waste to a composting toilet (or tight tank) will remove the majority of pathogens from the wastewater. It will also remove most of the nitrogen and approximately half of the BOD and phosphorus thereby reducing nutrient loading to the water body. Use of a shallow trench to further treat graywater may provide additional reduction in pathogens, BOD, and phosphorus loading.

A more difficult scenario is presented by septic systems which are deemed failed because of inadequate separation to groundwater. Obviously, the best solution in this scenario is to create a mounded leaching system so that adequate separation can be maintained. Where small lot size prevents this, compromises must be made. Title 5 allows a reduction to a two foot separation distance to groundwater for combined (gray and black) wastewater for local upgrade approvals when no other option for siting the leaching system exists (310 CMR 15.415). In this extreme case, it seems much preferable to remove the toilet wastes (to composter or tight tank) and allow only graywater to be disposed of with reduced separation to groundwater. Use of a shallow trench system, preferably located in topsoil, to further treat graywater may provide additional reduction in pathogens, and secondarily in BOD and phosphorus loading. Or, use of a closed evapotranspiration bed and composter will provide a zero-discharge system. A similar scenario is presented by existing septic systems which have inadequate setbacks to drinking wells. The 1995 Title 5 allows a reduction to a 50 foot setback to a well for a combined wastewater disposal system. In the extreme case where a 50 foot setback cannot be met, or where hydrogeologic conditions warrant further protection, a closed graywater evapotranspiration bed and toilet composter zero-discharge system may be the best solution.

A last (and more pleasant) scenario is presented by the home owner who wishes to voluntarily install a composting toilet and graywater disposal system in the belief that these will benefit the environment. The beliefs that motivate this choice range from a desire to conserve water in general to a desire to protect our embayments and ponds from nutrient loading and to a desire to protect groundwater from various drinking water pollutants. How well will use of a composting toilet and graywater system achieve these goals? Installation of a composting toilet can reduce household water use by up to 40% (although this can also be achieved by using ultra low flush toilets). Removal of toilet waste from household wastewater will also significantly reduce pathogen and nitrogen loading to the disposal system (however, see discussion in the section on Composting Toilets as to if and when these systems truly qualify as nitrogen removal systems). Total phosphorus discharge may also be halved. This can result in significant improvements in groundwater quality beneath the disposal system.

Some Concluding Thoughts

Careful consideration needs to be given to the issue of whether the cost of construction of an innovative graywater disposal system equals the benefit received. Conventional leaching systems that meet the requirements of Title 5 probably renovate graywater fairly completely with the possible exception of removing phosphorus. Shallow trench systems, may provide additional treatment in summer months when effluent is applied in the topsoil but may not provide any treatment beyond that of a conventional system in winter months when effluent is applied in the deeper soil layers. Closed evapotranspiration systems must be very large to function effectively in our climate, are costly to build, and probably are cost effective only in the most extreme circumstances when other disposal alternatives are limited.

Innovative graywater systems have been proposed, and are often marketed, as a solution to protect our groundwater. Proponents make the argument that innovative graywater disposal will protect groundwater from pollution by pathogens and nutrients. They also propose that use of innovative graywater systems will benefit groundwater in other ways. Firstly, if composting toilets are used, less household water will be needed, resulting in less groundwater withdrawal. Secondly, if the groundwater withdrawn for household use is later disposed of as relatively clean graywater, it is argued that the groundwater has been recharged with clean, rather than polluted, water. Is the amount of water recharged from a graywater disposal system significant and does it provide a net benefit to groundwater?

Most of the graywater disposal systems discussed in this document are designed to rely largely on evaporation and evapotranspiration to dispose of graywater. This implies that these systems produce little to no recharge of graywater to the groundwater. Even if all the graywater discharged is recharged to groundwater, it still makes up only a tiny percentage of the total annual recharge to groundwater. Cape Cod receives about 40 inches of rain per year, about 18 inches of which is recharged to the groundwater (the rest being lost to evapotranspiration by natural plant communities during summer months). This results in a net recharge to the groundwater of about 488,800 gallons of water per acre per year (1.5 ft. of rain x 43,560 sf/acre= 65,340 ft³/acre X 7.481 gal/ft³= 488,808 gal/acre). A graywater disposal system which produces a net contribution to groundwater of 100 gal/acre/day results in 3650 gal/acre/year recharge to the groundwater. This constitutes less than 1% of the total recharge to the groundwater over that land surface.

Recently, proponents of composting toilets and innovative graywater systems have urged DEP and the state legislature to allow the use of graywater disposal systems (in repair situations) with minimal setbacks: a 1 foot vertical separation to groundwater and/or bedrock, a 5 foot horizontal separation to surface waters, wetlands and other environmentally sensitive areas, and a 50 foot horizontal separation to surface drinking waters. Given the information we know about graywater, and the present lack of information about the presence or absence of viruses in graywater, we believe that these setbacks are not sufficiently restrictive to protect public health.

Progress Report – Graywater Recycling Septic System

Rarely a week goes by when we don’t receive an inquiry on how our wastewater recycling garden is working. Many of you know that under the Wellfleet Harbor Project, our Department installed a graywater recycling “garden” in a situation where no discharge on the lot was allowed. The crude schemata of the system is presented in Figure 1. Under guidance from a consulting company, the system was designed to collect the graywater from the house into a tight tank. Graywater was subsequently pumped into a lined planter bed (50′ x 12.5′) when the float switch was activated. The water level in the planter bed was prevented from rising too high by a relief system that allowed it to drain back to the tight tank. The environmental consultant supplied the design specifications and oversaw the construction of the system.

If the old expression is true that “you learn best from your mistakes” we are now much wiser from what we learned here. Foremost, we learned that the loading rate to this system grossly exceeded the ability of the system to remove the water through evapotranspiration. In our system, the three bedroom home (assumed 330 gallons/day), served by a composting toilet (assumed reduction in flow of 40%), would require a graywater system that would be able to handle 198 gallons/day. The graywater bed constructed provides approximately 625 ft2 of application area. We now understand this application rate to the garden is far greater than what can be evapotranspired. This system received very little use in the first year of installation, yet had consistent difficulty in eliminating the rainwater and the limited input from the house.

A second design flaw in our system was the system of effluent distribution to the bed. In the scenario where the bed is full, and it rains while graywater from the house is also entering the pump chamber, the pump continually runs, pumping “against the tide”. This constant wetting and production of saturated conditions is not conducive to evapotranspiration in the bed. We believe that this problem could be rectified by using a timer-controlled dosing, with adequate storage design to accommodate flows from the house and reasonable rain events. Even during drier periods, the timed-dosed system would provide better air movement through the system as opposed to the demand-dosed system that only provides large pulses of water during peak usage.

In addition, we now understand that better precautions to shed rainwater off of the top of the bed could have been employed, such as a loam and a seeded crown. “Dust mulching”, that phenomena purportedly responsible for sealing the sand and making it less permeable, was totally inadequate in this situation. After taking before and after-rainfall measurements in the bed after the system had been in for one year indicated that nearly all the rain permeates the crown of the system. The Wellfleet system is presently shut down until we can make the necessary adjustments. In the spring, we will also be introducing additional plantings and continuing to monitor the ability of this system to evapotranspire graywater.

A frank assessment of this technology suggests that significant refinements are necessary before it should find widespread use in Massachusetts. There are only a few, examples of its satisfactory performance in the state. Design guidance from reputable firms with long-standing experience is difficult to find. Although this technology, perhaps above all others, tempts the imagination with thoughts that our wastewater can be renovated, immediately recycled to produce greenery, and negate the need for any discharge, there are still many factors that must be considered. In particular, total evapotranspiration systems must be highly managed by individuals who understand the complexities of the soil systems used, the tolerances of the plants employed, and the characteristics of the wastewater applied. In situations, such as the one in Wellfleet, where there is a recycling of graywater, long term issues of salt buildup and plant tolerances must be adequately addressed. While not discounting the eventual development and use of this technology in Barnstable County, we would caution individuals contemplating total evapotranspiration beds to consult individuals with proven track records. Less critical, but still requiring significant expertise, are those situations where shallow soil horizons are used to receive graywater and make some use of the evapotranspirative qualities of overlying vegetation.

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