Introduction
Minnesota has a long history of providing wastewater treatment for clustered residential developments. In general, that has involved the installation of collector systems to solve existing problems. The best examples of these systems are found in lakeshore areas. Initially, most systems provided sewage treatment for resorts, where groups of cabins or lodges were hooked together by a sewer line delivering septic tank effluent to a soil-based treatment system. These systems have not been installed in new residential developments.
Some municipalities and small communities need to upgrade their wastewater treatment systems. Others are considering a cluster design for new residential developments. Local officials must decide what kind of wastewater treatment system to use. Until recently local officials had to choose from either a municipal wastewater treatment plant or a decentralized approach utilizing septic tanks and drainfields. There are now additional options available when using a decentralized approach. These alternatives include aerobic tanks, sand filters and constructed wetlands. Local officials need to review and evaluate their options carefully before selecting a specific system — including alternative systems — because the same approach won’t work in every case.
Currently, these alternative systems typically provide pretreatment to septic tank effluent before being discharged to a drainfield. To use these alternatives, more than the usual amount of long-term monitoring will be necessary to ensure that these systems consistently meet the operating standards claimed by manufacturers and proponents. From both a surface and groundwater perspective, soil-based treatment systems — if properly sited, installed, and maintained — can offer a high degree of protection and reliability.
In general, alternative systems serving clustered developments require more monitoring than systems that use septic tanks for pretreatment. Usually, alternative systems require additional pumps and sewage tanks, which results in extra maintenance. That’s why the organizations in charge of operating these systems need to be fiscally competent. While clustering has the potential to make operation and maintenance easier for an individual homeowner, a detailed plan for a development must be written and followed consistently. If that is not done, treatment will be less effective and there will be a greater negative impact on water resources.
Once the decision is made to use a cluster design, there are a number of factors to consider before choosing the appropriate wastewater treatment system.
Design and Siting Considerations
To be cost-effective and provide acceptable sewage treatment, the following factors must be addressed before choosing a system type and design:
- where the wastewater will be discharged to the environment;
- the type of collector sewer used;
- the estimated volume of flow (a number used to design the final treatment system);
- site characteristics (including the land footprint and projected future use);
- system reliability and monitoring;
- system maintenance and personnel requirements;
- adaptability to changes in system operation.
Minnesota Pollution Control Agency Permits
When wastewater is discharged to the surface or ground waters of the state, a National Pollutant Discharge Elimination System (NPDES) or a State Disposal System (SDS) permit is required. These permits detail the wastewater source, types of requirements for discharge, the amount of monitoring necessary, and the minimum level of treatment required. The Minnesota Pollution Control (MPCA) issues and administers both of these permits. Effluent limits are developed to protect water quality standards and the designated uses of waters. Both permits require monitoring to ensure the system is meeting the assigned effluent limitations.
When wastewater is discharged to the ground water via the ground’s surface a State Disposal System (SDS) permit is required. Additionally, if the discharge to the ground water is via the subsurface and over 10,000 gallons per day (gpd) an SDS permits is required. Local permits are required if the volume of wastewater discharged to the subsurface is less than 10,000 gpd. Future rules regarding class V injection wells, defined as any system that serves over twenty people, may impact the permitting of systems used in residential clusters.
An SDS permit requires ground water monitoring to demonstrate that drinking water standards are being met at the property boundary. If the system includes a licensed facility, such as a resort, mobile home park, hospital, retirement facility, etc., the Minnesota Department of Health (MDH) must also review the plan. The permit’s terms and conditions will vary depending on the ultimate disposal location of the treated wastewater.
If the discharge is to surface water, effluent limitations will be specified within an NPDES permit to protect water quality standards and the designated uses of the waters of the state. If the discharge is to ground water, the permit applicant will be required to meet drinking water standards at the property boundary. In both cases the permit will include monitoring of the effluent to ensure that standards are being met and to demonstrate that the system is operating efficiently.
To obtain an NPDES or an SDS permit, a permit application must be submitted to the MPCA at least 180 days prior to starting construction of the wastewater treatment facility.
Site Characteristics
There are several factors that should be considered when planning a wastewater treatment system that serves a cluster development and discharges to groundwater. The first is a general assessment of the suitability of a site’s geology and soil. Existing water table elevations, shallow aquifers, land slope, soil texture, and permeability must all be evaluated. In sensitive areas, additional treatment of the sewage effluent will be required. Site soil type and landscape position also need to be identified.
Soil type and wastewater flow determine the size of the system. The size and location of the soil treatment unit is determined by the estimated daily sewage flow and a sizing factor based on soil texture and permeability. Although not required, it is good planning practice to make sure that there is a secondary treatment site of equal size available. In the case of larger systems, those over 10,000 gpd, it is wise to be able to accommodate 2.5 times the estimated volume of flow. Providing additional area allows maximum operational flexibility and leaves room for future expansion.
There are a number of siting factors that can have a long-term impact on the operation and use of the system. Road and sewer development need to be coordinated with system siting and construction, for example. The collector sewer needs to conform with appropriate design standards. Location of the sewage treatment site needs to fit with the overall physical plan of the development. Areas reserved for future development need to be clearly identified. And the proposed sewage site needs to fit with existing plans for open space and buffers around a development’s residences (see Figure 1).
Estimated Daily Sewage Flow
Once site characteristics have been defined, an estimate can be made of the volume of sewage flow from a development. There is no simple recipe to follow when estimating such flows. It’s as much an art as it is a science. However, because the estimates of flow volume will greatly influence the type of system selected and how well that system performs, it’s important that system designers and community decision-makers be as accurate as possible.
The regulatory agencies, MPCA and MDH, play a major role in estimating flow volumes. If the agencies choose to continue with the current approach, considered to have a large safety factor built in, then minimal deviation from the current conservative estimates spelled out in Minnesota Rules Chapter 7080 should be used (Table 1). Currently, any reduction in flows from Chapter 7080 requires approval by the permitting authority.
Oversizing systems can have positive and negative results, depending on the final treatment system selected. For example, if a package plant (a non-soil-based treatment unit consisting of an aerobic tank followed by a chlorination process) provides wastewater treatment, oversizing leads to increased costs and lower operational efficiency.
On the other hand, some oversizing is desirable for soil-based treatment systems. Oversizing allows a treatment system’s parts to be rested periodically, creating more flexible operation and extending system life. From a regulatory view, oversizing also reduces the need for monitoring and maintenance. Both of these are positives for individual systems, where it is hard to get individuals to perform such simple maintenance tasks as the regular cleaning of septic tanks.
Flow is a critical piece of the puzzle. Keep in mind that basic decisions made early in designing a wastewater system carry through the construction and operating phases, and can have a large impact on system performance.
When estimating flows, it is important to strike a balance among three considerations — the desired treatment, the level of monitoring, and costs.
System Monitoring
In cluster wastewater systems, there is more focus on flexible operation and a greater need to monitor how well a system is doing. Monitoring adds an additional burden, to the owner-operator as well as the regulatory agency, because of the need to track, evaluate and change (or add to) a system based on its operating record.
System Types
After identifying the site and flow characteristics, the type of system can be selected. There is a wide variety of choices and they all offer advantages and disadvantages. The key is understanding each system's requirements and having a plan in place that will ensure the system's long-term operation. In looking at the available treatment options, it is necessary to discuss how they fit into a development plan, and where they should be used. It is important to note that all the systems described below require pretreatment, either through septic tanks or some other kind of sewage tank.
- Sub-surface Systems
· · · Community Drainfields
For individual sewage treatment systems not limited by soil conditions, the most commonly used unit is trenches. A drainfield trench is constructed by making a level excavation 18-36 inches deep. Clean rock is placed in the bottom of the excavation to a depth of 12-24 inches; then, a four-inch diameter distribution pipe, using one pipe per trench, is placed on the rock and covered with soil (Figure 2). Pipe or chamber systems without gravel can be used as substitutes for the rock. Treatment occurs in the natural soil through interrelated physical, chemical, and biological processes. Special siting considerations for trench systems include:
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| trenches need to be installed on a site's contour with the excavation depth limited by saturated soil or bedrock;
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| a minimum of 10 feet on center must be maintained between trenches;
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| the site must be large enough to accommodate a series of trenches laid along the natural slope. |
· · · Soil Treatment Mounds
In areas where limiting soil conditions do not allow the installation of sewage treatment trenches, mounds are an option. They are constructed with a layer of clean sand and leveled with a foot-deep rock layer before being covered by soil (see Figure 3). Special siting and construction considerations for cluster mound systems are:
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| the configuration needs to be a long, narrow rectangle;
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| mounds need to be installed on a site’s contour with the special consideration that they don’t act as dams for surface or subsurface flow across the site;
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| if more than one mound is required (which is usually the case), there must be adequate distance between them to allow for construction and to assure they do not interfere with one another hydraulically. |
· · · Constructed Wetland Systems
Constructed wetlands treat wastewater by bacterial decomposition, settling, and filtering (see Figure 4). As in tank designs, bacteria break down organic matter in the wastewater, both aerobically and anaerobically. Oxygen for aerobic decomposition is supplied by the plants growing in the wetland. Solids are filtered and finally settle out of the wastewater within the wetland. After about two weeks in the wetland, effluent is usually discharged by gravity to an unlined wetland bed.
If these systems discharge effluent to surface ditches, they require a National Pollutant Discharge Elimination System (NPDES) permit. In theory, any wetland design could incorporate a soil treatment system for final effluent treatment, but since the wetland itself takes up a lot of space, communities are unlikely to construct a soil treatment system in addition to the wetland.
· · · Sand Filters
The sand filter uses sand, like a mound in a box, as a medium for treating wastewater. This system has been used with great success for over 100 years and there is a large amount of information available about design and applications (see Figure 5).
Wastewater should be introduced by pressure distribution. The goal is to load the system as evenly as possible over the filter surface. This is best accomplished by using a pump to put the wastewater under pressure inside the pipe. This allows the waste to move through the filter at a rate that maximizes treatment. This system's treatment mechanisms are physical filtering and ion exchange. A properly operating sand filter should produce high quality wastewater.
· · · Drip Irrigation
This soil-based treatment system has been tested and used extensively in the southern United States. It uses small diameter tubing and a series of emitters to apply wastewater to the soil's upper layers (see Figure 6). By applying small amounts of effluent over a large area, evaporation is maximized, as is plants' ability to take up water and nutrients. The system is slightly larger than a conventional trench system. Although adding the effluent slowly over a large area increases treatment efficiency, lines freezing in winter can be a problem.
- Above-surface Systems
· · · Aerobic Tanks and Package Plants
Aerobic tanks treat wastewater far better than conventional septic tanks. This is due to the oxygen that is added to the liquid in the tank (see Figure 7). Aerobic tanks are, however, considerably more complicated to design, construct and maintain than septic tanks.
Aerobic tanks are available in residential or small-community sizes. In either case, these tanks require more maintenance than conventional septic tanks. If problems arise with the supply of air to the bacteria, an aerobic tank loses all its effectiveness. If there are problems with settling (more likely in these designs than with conventional tanks), there will be problems in the soil treatment system. It's critical that aerobic tanks be monitored regularly and repaired as needed.
For community aerobic tanks, there is a single location that needs checking and maintenance. Individual aerobic tanks provide multiple opportunities for problems and each one must be inspected as frequently as larger tanks. The aerobic tanks serving individual residences contain both the aeration and settling areas within the same tank. Since the discharge is to the soil there is no disinfection.
Package plants for small communities usually consist of an aeration tank followed by a settling tank and some type of disinfection or chlorination unit that treats the water before discharge.
· · · Spray Irrigation
Spray irrigation uses both biological and chemical processes to treat wastewater. The pretreated and often disinfected wastewater is applied at low rates to agricultural or wooded areas.
A spray irrigation system often consists of a septic tank (that provides a highly pretreated effluent), a sand filter and a disinfection unit within a spray application site. The final product is applied to the spray field through a conventional sprinkler system (see Figure 8).
Site suitability is determined by soil permeability, the depth to saturated soil or bedrock, the availability of a buffer zone, and land slope. For proper treatment of wastewater, the soil must remain unsaturated, just as it does in subsurface systems.
Compared to other wastewater treatment alternatives, spray irrigation systems require more land. That's why they may be best suited for recreational areas (such as golf courses) and agricultural land.
System Costs
Estimates should be made of a system's capital costs and its operational costs over its expected lifetime. Capital costs include land, equipment (tanks, pumps, rock, etc.) and construction. Operational costs include electricity, pump replacement, repairs, and such routine maintenance as the periodic cleaning of septic tanks or the replacement of sand in sand filters.
It is difficult to know whether one system is better than another. That's because any comparison depends on numerous factors, including how flows are estimated and whether research will confirm that less soil treatment area is needed for effluent that is largely pretreated. Other important considerations affecting comparisons are the specific site conditions, a site's slope and the location of individual lots.
It currently appears that the standards for soil treatment units contained in Minnesota Rules Chapter 7080 are cost-effective at flows of 5,000 gpd and less. For flows between 5,000 and 15,000 gpd, the least costly system is a series of individual septic tanks (one for each residence) connected to a communal drainfield or mound system. Sand filters, aerobics tanks and package plants become more advantageous, especially if a 50 percent reduction in the size of the soil treatment area is allowed. If there is plenty of low-cost land available, spray irrigation becomes a viable, cost-effective system. For flows over 15,000 gpd municipal wastewater treatment systems such as waste stabilization ponds and mechanical treatment plants start becoming cost-effective depending on the individual situation.
