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4.3.5 Subsurface Wastewater Infiltration Systems
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4.3.5 Subsurface Wastewater Infiltration Systems

Description:
Description:
Information

As the wastewater percolates or moves down through the soil, a variety of complex physical, biological, and chemical processes combine to provide additional treatment. The primary infiltrative surface is the bottom and sidewalls of the excavation. Perforated pipe or subsurface dripline is installed to distribute the wastewater over the infiltration surface. In a conventional system, a porous medium, typically gravel or crushed rock, is placed in the excavation below and around the distribution piping to support the pipe and spread the localized flow from the distribution pipes across the excavated trench or bed. A permeable geotextile fabric or other suitable material is laid over the porous medium before the excavation is backfilled to prevent fine particles in the backfill soil from migrating into the porous medium. Particles in the wastewater are filtered by, adhere to, or chemically bond or react to the soil. Bacteria and other organisms in the soil consume the residual organic matter in the wastewater and perform most of the treatment.

 

Design Configuration

 

There are various SWIS configurations that are well suited for small village wastewater systems.

  • Wastewater can be distributed to the system by gravity flow or dosed distribution.
  • Infiltration surfaces may be created in natural soil or imported fill material. The key difference between the two approaches is that a secondary infiltrative surface is created at the fill-natural soil interface.
  • The infiltration surface can be designed in in-ground trenches, at grade level, or elevated in a mound.

 

Design Criteria:

 

General:

 

  • The leach field must not subject to traffic, heavy equipment, nor used as pasture for domesticated animals.
  • The site must have unsaturated vadose zone to processing the effluent. Aerobic conditions shall be present beneath the infiltration surface.
  • It is important to maintain adequate distances from overly steep slopes, landslides, and/or shallow groundwater and bed rock. It is also important to maintain minimum distances from manmade features, including water supply wells and springs, waterlines, and building foundations.
  • The minimum vertical separation between the bottom of infiltration surface and groundwater depends on the influent quality into the system. If wastewater is primarily treated (BOD greater than 30 mg/L), the minimum separation should be 1m. For treated wastewater (BOD, TSS and < 30 mg/L and total nitrogen (TN) < 25 mg/L), the minimum separation is 0.6 meters.
  • Any sloping site that is subject to frequent inundation during prolonged rainfall should be considered a candidate for upslope curtain drains to maintain unsaturated conditions in the vadose zone.
  • Using sidewall area as an active infiltration surface should be avoided.
  • Multiple trenches to provide periodic resting, standby capacity, and space for future repairs or replacement should be designed. Normally, the SWISs should be designed to have a total hydraulic capacity of 100 to 200 percent of the design flow to allow periodic resting of a portion of the SWISs.
  • The minimum area of infiltration surface needed depends upon the daily design rate and the hydraulic loading rate.

 

Size of trenches/beds:

 

  • The maximum depth of the infiltration surface should not exceed 0.9-1.2 m below final grade to adequately reaerate the soil. The infiltrative surface depth should be less in slowly permeable soils. In cold climates, a minimum depth of 0.3 to 0.6 m may be necessary to protect against freezing.
  • Infiltration surface clogging and the resulting loss of infiltrative capacity are less where the infiltration surface is narrow. Therefore, trenches perform better than beds. Typical trench widths range from 0.3 m to 0.9 m, and bed widths greater than 3 m are not recommended. When slope is greater than 15%, trench widths should be reduced to 0.3 – 0.5 m.
  • The trench length should not be greater than 30 m for gravity flow systems, and shorter trench length (less than 15 m) are more suitable for pressurized systems.
  • The minimum height of sidewalls should encase the distribution piping or meet peak flow storage requirements.
  • Spacing between adjacent trenches should be at least 1 to 2 m. In sloping sites, the spacing should be increased. When slope is greater than 15%, the horizontal separation need to be increased to 3 m.
  • The diameter of laterals should not be greater than 50 mm or less than 38 mm to allow for cleaning.

 

Distribution systems:

Soil type

Minimum dosing frequency

Coarse sand, gravels, sand mounds, etc.

Timed dosing

Medium sand, fine sand, loamy sand

4 times per day

Sandy loam, loam, silt loam, clay loam

2 times per day

Well structured sandy clay, silty clay or clay

4 times per day

(Source: The World Bank, 2009)

  • When slope is greater than 25%, pressurized systems with narrow trenches are recommended.

 

Backfilling

 

  • Natural soil is typically used for backfilling, and the surface of the backfill is usually slightly mounded and seeded with grass.
  • If the trenches are at grade level, the backfill layer should have at least a thickness of 30 cm.

 

Special conditions

 

Special conditions

Strategies

Highly permeable soil

(Sites with gravel or gravelly sand and soils with high coarse fragment content)
  • Increasing the vertical separation.
  • Increasing the dosing frequency and decreasing the dose volume.
  • Creating uniform distribution of effluent in each dose.
  • Reducing the hydraulic loading rate, resulting in larger treatment areas.
  • Using soil treatment components that include a soil layer of reduced hydraulic capacity such as mounds or sand lined trenches.

Very low permeability

(sites with clay loams and clay textured soils, and soils with unfavorable structure and consistence)
  • Reducing the hydraulic loading rate, resulting in larger treatment areas.
  • Creating uniform distribution of effluent in each dose.
  • Increasing the dosing frequency and decreasing the dose volume.
  • Improving the quality of effluent applied to the very low permeability soils.
  • Decreasing the linear loading rate.

Special soils

(prismatic, columnar or platy soil structures)

Fully understand the soils on the site by increasing the number of test pits and completing additional soil testing.

(Source: Government of Saskatchewan, 2018)

 

  1. Placement of the infiltration surface. The vertical separation between the infiltration surface and the limiting condition (shallow groundwater, rock layer or other hydraulically restrictive horizon) depends on the hydraulic loading rate, effluent quality, distribution methods, and soil characteristics. Uniform, frequent dosing (more than 12 times/day) in coarser soils may reduce the needed separation distance (EPA, 2002). Normally, in-ground or at-grade level trenches are used. At sites with high groundwater table, shallow soil depth, or shallow bedrock, a mound can be constructed using imported materials (e.g., soil, or sand) to raise the infiltration surface and increase the vertical separation as needed.

 

  1. Flow distribution. After pretreated in a septic tank, the effluent flows into the leach field either by gravity or pressurized system for disposal. Uniform distribution of effluent results in more complete utilization of the infiltration surface.

 

  • Gravity flow. Gravity flow can be applied in simple topography areas where there is sufficient elevation difference between the septic tank outlet and the leach field. Although simple and inexpensive, it is the least efficient method of distribution (EPA, 2002). Most of the effluent infiltrates into the soil at the beginning of the trenches, which results in uneven distribution over the infiltration surface and may further lead to localized overloading and system failure.

 

  • Pressured flow (dosed distribution). When the site has a flat topography, or the infiltration surface has a higher elevation than the septic tank outlet (such as a mound system), the effluent will be pumped into the distribution lines at certain time intervals instead of by gravity. The “dose-and-rest” cycles allow the soil around the trenches to reaerate, which reduces the rate of soil clogging and more effectively maintains the unsaturated conditions in the subsoil. The pressure dosed pipe typically has a smaller diameter and smaller perforations than the gravity flow pipe.

 

Dosing configuration. Timed dosing with peak flow storage arrangement is recommended. If applying on-demand dosing, the effluent discharge will be concentrated around the peak time of wastewater generation, which may result in ponding. The dosing volume shall be at least 67% of the daily design flow.

 

  1. The pump system, if applied, will include high water level emergency alarms to alert operators to potential problems within the system when water level rise above the normal operating level. The alarm shall be connected to a different electrical circuit from the pump.

 

  1. Trench configuration. Trenches may be constructed in parallel, sequential, or serial configuration (Figure 4.13 and Table 4.4). The choice of a particular configuration depends on the topography and soil conditions.

Figure 4.13 Schematic diagram of different trench configurations

(Source: WERF, 2010)

 

Table 4.10 Comparison among different trench configurations

 

Parallel trenches

Sequential trenches

Serial trenches

Process

Effluent from the septic tank first flows into a distribution box. Once the distribution box fills up to the outlets, equal amounts of effluent move out to each trench

The effluent flows directly into the first trench with the highest elevation. As the trench gradually fills up to a predetermine level, effluent then passes on to the next lower trench in sequence.

Effluent flows into the first trench and will not enter the next trench until the former trench is fully ponded.

Topography

Sites with flat topography

Sloping sites

Sloping sites

Trench length

Each trench has the same length and at the same elevation.

Equal length of each trench is not required.

Equal length of each trench is not required.

Operation

Uneven distribution often occurs. Tipping and weir distribution device options are sometimes used to improve water distribution.

One trench must be fully ponded before the effluent flows into the next trench. Individual trenches can be taken out of service for resting.

As the trenches are continuously ponded with effluent, the system will be clogged more quickly, as the infiltration surfaces cannot regenerate their infiltrative capacity.

 

  1. Conventional gravel trench and gravelless trench. In a conventional gravel system, perforated pipes are surrounded by washed gravels with uniform size. A geofabric or similar materials is placed on the top to keep small particles from entering and clogging the clean gravel layer. The void spaces among gravels provide storage area for effluent and avoid effluent inundating the soil too quickly. The best size of gravels is between 1.9 cm and 6.3 cm. Due to the combined effects of soil compaction and biomat development in void spaces among gravels, the infiltration rate of the gravel system is restricted to some extent.

 

Chamber system with no bottoms and plastic chamber sidewalls is a typical gravelless system. As effluent directly contact with soils, the infiltration area can be over 50% higher than the gravel system, where gravels somehow mask the soil surface. The chamber system also has a higher storage capacity than gravel system.

 

Therefore, gravel system is suitable for sites where large parcel of lands and gravels are locally available. Transportation of gravels will increase the capital costs. In areas where gravel is scarce and available land is limited, the chamber system is more suitable, as the materials are easy to deliver and the system requires less footprint.

 

Operation and Maintenance:

 

SWISs require little operator intervention. Typical operation and maintenance activities include:

 

  • Regular inspection on the function of pump, switches, and timers for pressure-dosed systems.
  • Direct wastewater to standby trenches to rest operating trenches semi-annually or annually. Summer is a good time to rest a trench as warmer soil temperatures will shorten the time needed for treatment capacity restoration.
  • Regular inspection on the SWIS area to observe surface ponding or other signs of depression or damage.

 

To protect the SWISs, trees or deep-rooted plants should be kept away from the leach field. Construction of irrigation systems in the leach field is not allowed and no irrigation water should not run towards the leach field.  

 

Capital Cost and O&M Cost:

 

  • Generally, subsurface wastewater infiltration systems are passive, effective, and inexpensive treatment systems.
  • The capital cost and operational costs are low. They generally require an expert to evaluate soils, design the system and supervise the construction.

 

Applicability:

 

  • Suitable for areas with large amount of available lands;
  • Not applicable in environmental protection sensitive areas and areas with high level of groundwater table;
  • The system loading is low and it is not suitable in areas with dense population and large amount of wastewater.

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