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Composting Toilets

Recently, composting toilets have been promoted as solution to groundwater pollution problems caused by bacteria, viruses, and nutrients such as nitrogen and phosphorus. In addition, the alternative technologies portion of the revised Title 5 which took effect November 10, 1994 has eased the regulatory climate regarding composting toilets significantly. These two factors make it likely that Boards of Health will be asked to consider an increasing number of applications for the use of composting toilets.

Under the 1978 code, composting toilets were allowed with approval by DEP but the owner was required to install an entire Title 5 septic system consisting of a septic tank, D-box and leaching facility to dispose of the dwelling’s graywater. The economics of installing a composting toilet plus a Title 5 system, combined with the lengthy approval process, made the installation of composting toilets unattractive except in the most extreme of circumstances.

Figure 1. General Schemata of a composting toilet configuration.

Under the revised Title 5 composting toilets are certified for remedial use and can also be used for new construction where a system in full compliance with Title 5 could be otherwise be installed. In addition, the new Title 5 has eliminated the requirement for a complete Title 5 system for graywater. Under the new Title 5, for new construction, the leaching facility may be downsized to 60% of the dwelling’s design flow and a filter system may be used in place of the septic tank for systems where there is no discharge of garbage grinder waste or liquid by-product from the composter to the graywater system. For remedial use, an existing cesspool may be used as the graywater leaching pit, provided it is not located in groundwater and it meets the effluent loading requirements of Title 5. These changes to the code make the use of composting toilets more economically attractive.

This chapter explores how composting toilets work, end products produced, how well these systems remove nutrients, and offers suggestions to homeowners and Boards of Health who are considering use of these systems.

How do Composting Toilets Work?

Composting toilets are contained waste treatment systems which use natural biological decomposition to convert toilet wastes into water vapor, carbon dioxide, and a stable compost-like end product. The decomposition process is accomplished by aerobic (oxygen-using) bacteria and fungi. The complex population of microorganisms in the composting material make conditions unfavorable for the growth of disease-causing organisms which can be present in human waste. Pathogenic organisms die off or are consumed by the composting organisms as long as the composting process is proceeding normally and has adequate time to work.

In order to produce a thoroughly decomposed compost product, three conditions are essential.

1. The process must remain aerobic. The microorganisms which decompose the waste need oxygen to flourish. Aerobic conditions are maintained by mixing the pile and by controlling moisture.

2. The compost must be maintained at the correct moisture content. If the compost becomes too dry, decomposition will not occur. If the compost is too wet, it will not remain aerobic and decomposition will cease. Humans excrete a much higher volume of liquid than solid each day. This excess liquid must be managed to ensure the composting waste does not become too wet. Excess liquid is managed either by evaporating it off using fans and heater units inside the compost chamber, or is collected at the bottom of the composter unit where it must be disposed of in an acceptable manner. Bulking agents, such as wood shavings, are also commonly added on a regular basis to ensure proper drainage of liquids from the compost and to increase the evaporative surface area.

3. Temperatures must be maintained above 60o F for composting to proceed effectively. At lower temperatures bacterial activity is inhibited and the composting process slows. Temperature can be controlled by maintaining the composting unit at room temperatures above 60o F or by placement of heating units inside the composter unit (units with larger storage volumes can operate at lower temperatures because composting will still proceed slowly and these units have the storage capacity to accommodate a slow composting process.

Types of Composting Toilets; Pros and Cons

There are two types of composting toilets generally available for residential homes. The first type are relatively large, bi-level, watertight containers equipped with a chute that connects the toilet receptacle to the composting unit located in the basement. The composting unit often has an inclined floor where solid waste decomposes and slides to the lower end as new waste enters at the upper end. Excess liquid is drained to the lowest part of the tank where it is either evaporated or collected. Compared to self-contained units (below), bi-level composters have a large compost volume and long retention time. Thus, the composting process is more stable than in smaller units, is better able to cope with peak loads, and can withstand intermittent or seasonal use. Finished compost generally need only be emptied annually or once every several years. The best known bi-level type composter is the Clivus MultrumTM.

The second type of composting toilet are smaller units in which the toilet receptacle and composting tank comprise a single self-contained unit located in the bathroom. These units have traditionally been installed at intermittently-used vacation homes but also have been marketed for year round residential use. The most well known of these units are marketed by BioLet USA, Inc. (“BioLet”), Sun-Mar Corp. (“Sun-Mar”), and SanCor Industries Ltd. (“Envirolet”).

Figure 2 . Schemata of Biolet® XL recently approved by DEP

Smaller, self-contained units are less expensive, easier to install and can usually be retrofitted into existing dwellings. However, the composting unit is smaller than in bi-level units. Smaller composting units have a shorter residence time with the result that waste may not be fully decomposed before it is discharged, the unit must be emptied more frequently and there is greater potential (and less storage capacity) for liquid accumulation if the unit is overused. Most of these units have liquid collection chambers. If the liquid is not piped to the graywater leaching system, the chamber must be emptied and disposed of properly on a regular basis. Some newer designs incorporate mechanical aeration and heaters so that the composter operates at temperatures of 80-110o F and the composting process proceeds more rapidly. These units are intended to evaporate all excess liquid so that there is no discharge other than finished compost. Based on owner operation manuals it appears that compost must be removed from these units several times per year. It is also questionable whether these smaller units meet the requirements of Title 5, section 15.289(3)(a), which requires that composting toilets be designed to store composted and compostable solids for at least two years. Recently (March 1997), the BioLet XL (Figure 2) composter (which is equipped with heater and fan units) has received Provisional and Remedial Use approval from DEP specifically exempting the unit from the requirement that it be able to store composted solids for two years.It is unknown whether other units will also be exempted from this requirement in the future.

Composting Toilet

All small composting units intended for regular, year-round use should be equipped with several devices to ensure that they function properly. They must be equipped with an adjustable thermostat/heater unit so that evaporative capacity can be adjusted based on use of the unit. This device is essential to ensure proper moisture content of the compost and to prevent excess liquid accumulation. Units must be vented and equipped with an automatic fan which acts to aerate the compost, and also prevents excess liquid accumulation. Units should also be equipped with an automatic mixer which is used to increase aeration and speed the decomposition process.

It is important to note that small composter units require a fairly high degree of owner involvement . The owner must monitor the compost carefully for the correct degree of moisture and empty excess liquid as it accumulates. Compost must be disposed of on a regular basis. It should also be noted that these units may not function well in extended power outages when fans, heaters and mechanical aerators become inoperable.

Solid and Liquid End Products

All composting units produce a solid compost-like end product and most produce a liquid end product (at least occasionally). These products have the potential to contain pathogens and DEP has strict requirements for their handling and disposal. Household graywater must also be disposed of, generally through conventional Title 5 leaching components. Recently, shallow trench systems with drip-emitter tubing have been proposed for graywater discharge during warmer months when plants can take up the water through their root systems. This type of system must be approved by DEP on a case-by-case basis.

Solid End Products

When solid wastes are completely decomposed, the average person produces 2-3 gal (0.25-0.4 cubic ft.) of compost per year. The compost produced by a Clivus Multrum (the only system for which we have seen data) is approximately 2.5% total nitrogen and has a nitrogen:phosphorus:potassium (N:P:K) ratio of 2.5% N:3.6% P:3.9% K. This compares to garden compost with a typical N:P:K of 2.5:1.5:1.4 or composted sewage sludge at 2.1:2.2:0.3. Nitrogen levels in all are comparable (note that typical lawn and garden fertilizer has an N:P:K of 10:10:10). However, approximately 70% of the nitrogen in the Clivus compost is present as organic nitrogen. This form of nitrogen is bioavailable for uptake by plants and soil microorganisms but will not readily leach to groundwater. As long as the compost is applied in the root zone it can serve as an excellent soil conditioner with minimal nitrogen impact. It should be noted that nitrogen content may potentially be higher in compost produced by small composting toilets. Because these toilets evaporate rather than drain excess liquid, the nitrogen present in urine is likely to become concentrated in the compost.

Phosphorus in the Clivus end-product is somewhat higher than in comparable compost products. However, the majority of the phosphorus is present as organic phosphorus, in which form it is unlikely to leach to groundwater but is bioavailable for uptake by plants.

Pathogenicity of properly decomposed compost appears to be low. Testing performed by Clivus indicates that solid end product in the Clivus composter ranges from 0-35 fecal coliform per gram of compost (septic tank sludge averages 100,000 FC/gram). The majority of bacteria present in the compost are those normally prevalent in soils. The presence of viruses in the compost has not been adequately investigated nor have we seen data for the bacterial content of compost produced by small composting toilets. DEP considers the solid end product to be potentially pathogenic and requires that it be disposed of in a manner such that it cannot be contacted by humans.

The revised Title 5 section 15.289 3(a) requires that solids produced by a composting toilet shall be disposed of either by burial on-site or in another manner and location approved by the Board of Health covered with a minimum of six inches of clean compacted soil, or be disposed of by a licensed septage hauler. If collected by a licensed septage hauler, the solids may be disposed of at a septage treatment plant, bagged and sent to SEMASS, or disposed of at a landfill.

Liquid End Products

Liquid end product routinely accumulates in bi-level composters such as the Clivus Multrum. It may also accumulate periodically in small self-contained composter units, although this usually indicates that the composter is overused or is not working optimally. This liquid end product is primarily urine which has filtered slowly through the composting waste. Limited information available from Clivus Multrum suggests that typical Clivus composter units at residential homes produce 1-2 liters of liquid end product per day.

Urine from healthy individuals does not contain fecal coliform or other bacteria. However, as the urine filters through the compost, it could potentially pick up fecal coliform or other pathogenic bacteria and viruses. Testing performed by Clivus indicates that liquid end product in the Clivus composter shows very low fecal coliform levels: frequently less than 10 FC/100 ml and almost always less than 1000 FC/100 ml. The presence of viruses in the liquid end product, however, has not been adequately investigated. DEP considers the liquid end product to be potentially pathogenic and it must be handled and disposed of in the same manner as conventional sewage.

The revised Title 5, section 15.289(3)(a), which took effect November 10, 1994 requires that if a composting toilet produces a liquid by-product then the liquid must be discharged through a graywater system that includes a septic tank and leaching system, or it must be removed by a licensed septage hauler and disposed of properly (other disposal options such as incorporation in a graywater recycling system will be approved by DEP on a case-by-case basis). All composter units should be installed so that they are accessible to a septage hauler for liquid removal.

It is also important to note that the liquid end product is very high in nitrogen. Data from Clivus Multrum indicates that their liquid end product ranges from 2000-10,000 mg total nitrogen per liter (typical septic tank effluent averages 60-90 mg N/L). This is because the liquid end-product is composed primarily of urine which has not been diluted with toilet flush water. The urea in urine is a nitrogenous compound and is the body’s main route for excretion of nitrogen. Humans excrete an average of 6 kg of nitrogen annually and 2/3-3/4 of this nitrogen is excreted in urine. Nitrogen in the liquid end product is present almost totally as nitrate which leaches readily to the groundwater. If the liquid end product is disposed of in the septic system the composting toilet is almost no different from a Title 5 septic system in terms of its contribution of nitrogen to groundwater. However, if the liquid end product is reclaimed for re-use and used appropriately it can be excellent fertilizer.

Do Composting Toilets Quality as Nutrient Removal Systems?

How environmentally friendly are composting toilets? Can they be used to alleviate eutrophication problems caused by nitrogen and phosphorus in our coastal waters and ponds?

It is important to recognize that composting toilets do very little to change the quantity of the major nutrients– nitrogen and phosphorus– in human waste. Composting toilets can reduce the volume of solid waste by turning it into compost. They can prevent the addition of pathogens to the groundwater by removing human waste from the septic system. They can reduce water consumption. But there is no net loss of nitrogen or phosphorus in a composting toilet; nitrogen and phosphorus are merely repackaged into different forms. In recognition of this fact, DEP has not issued nitrogen removal credits for any composting toilets to date.

At present there are three basic disposal options for wastes produced by composting toilets. Waste can be disposed of on site: compost is buried and liquid waste is disposed of in the graywater septic system. In this case there is no net removal of nitrogen or phosphorus and the composting toilet cannot be considered to meet the goal of nutrient removal. A second disposal option is that the solid and liquid waste is removed periodically by a licensed septage hauler and taken to the local septage treatment plant. In this case, nutrients have been removed off-site which may alleviate local nutrient loading problems. But, unless the septage treatment plant has advanced nutrient removal capabilities, there is still no net loss of nitrogen or phosphorus– they have merely been exported to have environmental impact elsewhere. A third disposal option is collection of the wastes by a licensed hauler who intends to market the products for use in place of commercial fertilizer. Although there is still no net loss of nutrients, the end products have been substituted for agricultural fertilizer which would have been used anyway, so this solution can be considered to be a net benefit and environmentally friendly. Recent conversations with a representative from Clivus New England indicate that Clivus hopes to develop the capability to remove solid and liquid wastes from all units it sells and installs. Clivus hopes to develop a central collection facility for these wastes and is working to obtain DEP approval for agricultural re-use of these products (however, this has not occurred as of June 1997). If this service does not become available, Boards of Health should recognize that composting toilets do not qualify as nitrogen removal systems.

Recommendations to Board of Health Who will be Approving Composting Toilets

Where is the installation of composting toilets most appropriate? In repair situations where minimal Title 5 setbacks to drinking water wells and watercourses cannot be met composting toilets may be a good solution since they remove human waste and potential pathogens from the septic system. Composting toilets may also be appropriate for repair situations in tight soils or other situations where water use must be minimized. Composting toilets can also be appropriate to remediate nutrient loading problems as long as adequate provisions are made for appropriate disposal of the end-products.

It is very important for Boards of Health to know that the end-products of the composting toilet will be properly disposed of. A maintenance contract between the owner and a licensed septage hauler for removal of wastes is essential. Because the end-products are potentially pathogenic, a licensed septage hauler should perform maintenance of the system and removal of end-products to reduce exposure of the owner and the public to potential pathogens. Most septage haulers, however, may have limited experience with composting toilets. The Board of Health may wish to require the owner to have an additional contract with the manufacturer or distributor for periodic maintenance to ensure that the system functions properly and that any problems are corrected quickly. For systems which are proposed to remediate nitrogen loading, the Board of Health will also want to make sure that the owner contracts with a septage hauler (or the composting toilet distributor) who agrees to take the end-products to a collection facility where they will be appropriately handled for re-use as fertilizer.

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