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Bark Bed

How a bark bed works

A bark bed is a relatively flat, soil infiltration area covered with bark or wood shreds. Effluent from the septic tank(s) flows into a dosing tank where it is then pumped via a pressure distribution system to the soil infiltration area. Effluent is then pumped through distribution pipes to either a chamber system or drainfield rock. The entire area is then covered with 18-24 inches of inches of bark or wood shreds. The bark covering allows good oxygen transfer to the effluent/soil interface which speeds the organic matter breakdown. The bark also keeps the infiltration area from freezing in northern climates and aids in effluent evaporation. Figure 2 shows a schematic of a bark bed.

Pretreatment

Figure 2. Schematic of bark bed.

Effluent entering a bark bed must be pretreated in a primary septic tank with a minimum 3-day HRT, the volume of the bulk tank, or 1000 gallons, whichever is greater. All bark bed systems require a second septic tank with a three-day HRT (for a total system HRT of six days) and commercial size effluent filter for treatment prior to distribution in a bark bed.

Infiltration area

Bark bed size is based on the infiltration capacity of the soil which is a function of soil type. In addition, treatment requires a minimum two-foot separation distance (soil depth) to groundwater or bedrock. This two-foot separation removes the remaining organic material and reduces nutrients in the wastewater and assures that the system functions hydraulically. (Note that household septic systems require a three-foot separation to groundwater or bedrock to treat human pathogens.)

The infiltration area can be located on previously farmed cropland, pasture, or wooded areas where trees and small vegetation have been removed. It is important that this location be uncompacted, natural soils to insure good soil infiltration. In addition, the bark bed should not be located within 100 feet of wells or sinkholes or 150 feet of any lakes streams or wetlands.

Bark bed sizing

A combination of organic (BOD5) and hydraulic loading are the basis for sizing the infiltration area. Table 2 shows the recommended size of infiltration area needed based on average effluent strength from the second septic tank of 750 mg/L BOD5. These sizing factors are based on the soil texture and the ability of the soils to breakdown the organic matter and are approximately six or more times larger that of typical home septic systems. Dividing the Loading Rate, found in Table 2, by the total daily wastewater volume per day will determine the size of the soil infiltration area. Note that these Loading Rates are based on the concentrations listed in Table 1. Additional wastewater treatment to reduce these concentrations, significant deviation in water usage, or additions of manure to the system (e.g. parlor waste), would change these factors.

Bark bed siting and layout

To perform properly, Bark Beds must be constructed with 0% slope in all directions. Due to construction practices and pumping requirements, the recommended maximum bed width is 30 feet and the maximum bed length is 220 feet. If there is a choice, the infiltration area should be narrow (10 feet wide) and long rather than wide and short. Multiple beds can be used to meet the infiltration area requirements but should be avoided if possible.

A backhoe is used to roughen the infiltration area.

If the area is level and covered with vegetation, the surface should be roughened with backhoe teeth to assure that the wastewater will not be impeded. If there is no naturally level area, the site should be excavated to achieve a 0% slope. No more than two feet of top soil should be cut, Figure 3. Filling in is not allowable as the natural soil structure is required for good infiltration. Soil surfaces should not be smeared or compacted during excavation. In addition, the design infiltration rate should match the exposed subsoils. (Often excavation could uncover different soils than the topsoil.) Do not select areas with more than 6% original slope. Berms are not necessary to keep the effluent inside the bark bed. However, runoff water from other areas should be excluded from the infiltration area using berms or site excavation. Large animals must be kept off the bark bed to avoid disturbing the bark or compacting the infiltration area. During and after construction, heavy traffic on the soil infiltration area should be avoided to minimize soil compaction.

Distribution system layout

Effluent is distributed to the soil infiltration area using a pressure distribution system which insures good distribution of the wastewater over the entire infiltration area. (Gravity distribution to the bed is not recommended.) Pump requirements and piping design must be sufficient to deliver two feet of head (water column) to the bark bed area after accounting for pressure losses for elevation and pipe friction.

A standard effluent pump is used to supply the pressure distribution system. The pump is set in the pump compartment of a two compartment tank or in a separate dosing tank sized for a one-day HRT or a minimum of 500 gallons. To avoid pumping solids into the infiltration area, the pump intake should be located a minimum of six inches off the bottom of the tank.

Figure 3. Bark bed layout on a hillsope.


Gravel, distribution pipe, geotextile fabric and bark media being installed.

The pump is controlled with a high-low float switch. Pump floats should be adjusted so the bark bed is dosed at least once per day to minimize system freeze up in very cold weather. The minimum dosing should be a volume of five times the distribution pipe volume. This minimum pumping volume insures even pressures and distribution throughout the infiltration area. A high alarm float must also be installed to warn of a system failure such as a pump failure or excess surface water entering the system. Details on pump and piping calculations can be found in Section 9 of the the University of Minnesota OSTP Manual.

Effluent distribution in the soil infiltration area can be done in two ways. Pressure distribution pipe can be laid on top of a gravel spreader or hung in a chamber designed specifically for wastewater distribution. A gravel spreader of drainfield rock must at least six inches deep and five feet wide. The distribution pipe is laid on top of the gravel spreader, covered in two inches of rock and then covered with synthetic landscape fabric to limit plugging of the gravel with the fines from the bark or wood shreds. When using a chamber system, the distribution pipe is suspended at the top of the chamber with plastic ties or mounted per the manufacture’s specifications. All pressure distribution pipe laterals must be placed at the same elevation for uniform distribution. If this is not possible, see Section 12 of the University of Minnesota OSTP Manual for specific design changes related to elevation differences in pressure distribution systems.

Each pressure distribution lateral will feed a soil infiltration area 10 ft wide (5 ft on each side of the pipe). As such, the minimum distance between lateral distribution lines is ten feet. Bark or woodchip covering must extend five feet from the centerline of the pipe in both directions and on the ends. Note that the five foot of covering on the ends of the laterals are included in the effective infiltration area of the bark bed.

The sizing of the pipe and pump is critical in the design and performance of a bark bed. The distribution laterals are fed by a manifold pipe. This manifold can feed the laterals from either the center or an end as shown in figures 4 and 5. The sizing of the manifold and other design parameters are based on the geometry of the distribution system.

Pipe sizing and hole spacing

Holes are drilled in distribution pipe.


Distribution pipe is hung in chamber.

Several factors are critical in determining the lateral pipe sizes, manifold size, and hole spacing and size in the lateral pipes. All of these parameters are interrelated and used to determine the size of the pump. In general, two-inch Schedule 40 PVC pipe is used with 1/4 inch holes drilled every five feet. (For designs with other hole spacings see Section 12 of the University of Minnesota OSTP Manual). Holes are drilled on the bottom side of the pipe when gravel spreaders are used. When using chambers, refer to the manufacturer specifications for hole location, making certain that some of the holes are on the bottom side of the pipe so the pipes drain completely after the pump shuts off. In addition, a 1/4 inch hole should be drilled into the end cap of each of the lateral pipes to release air pressure during filling of the pipe.

Figure 4. End manifold design.


Figure 5. Center manifold design.

To maintain good distribution along the length of a single distribution pipe, a maximum number of 22 quarter-inch holes can be drilled. Therefore, with a hole spacing of five feet the maximum length of any lateral pipe length is 110 (5 foot spacing x 22 holes.) Systems with a center manifold (Figure 5) can distribute effluent over a bed length of 220 feet (5 foot spacing x 22 holes x 2 directions).

The pressure in these lateral distribution pipes should be maintained at 2 feet of head (0.86 psi). With this pipe, hole sizing, and pressure, the flow rate through each hole is 1.04 gallons per minute and the velocity in the pipes will be greater than the minimum recommended velocity of two feet per second. To determine the total flow rate in the system, multiply the total number of holes in the system by 1.04.

Manifold sizing is based on the number and spacing of distribution laterals. Laterals are spaced at ten-foot intervals along the manifold. With only one distribution line there is no need for a manifold as the supply line will feed the lateral directly. With two laterals the length of the manifold pipe is ten feet. With three laterals, the manifold length is 20 feet. Manifold diameter should be the same size as supply line or larger. The supply line should enter the bottom of the manifold to allow for good drainback and can be placed at any location along the length of the manifold.

Pump sizing

Pumps are sized using the design flow rate and pressure. Pressure is required to evenly distribute the effluent to the infiltration area, account for elevation differences between the pump and the lateral distribution lines, and overcome friction losses in the piping system.

To provide even distribution in the distribution laterals two feet of head is recommended.

Pressure loss due to elevation differences is a function of the individual site layout. The pressure is the difference in elevation between the bottom of the pump and the distribution line.

Pressure loss due to friction in the supply line, manifold and laterals is a function of the flow rate, pipe diameters, and pipe lengths. Table 3 shows the friction loss in the main supply line (per 100 ft of length). Friction pressure losses in the manifold and lateral distribution lines are minimal and are taken into account with the hole sizing and spacing.

In addition to the friction losses along the length of the supply line pipe, there are additional friction losses in each pipe connection (elbows, joints, etc). These losses can be calculated individually (per pipe connection using various tables or charts) or can be estimated by adding 25% more to the length of supply line to account for these losses. Note that if the supply line is very short and there are several joints in the system this 25% increase may not account for all the friction losses. Conversely, if there are few joints and elbows with a very long supply line the friction losses may be significantly less than the 25% value.

Example 1: Pressure loss calculation.

Calculate the total head loss in a piping system that has a total flow rate of 50 gpm, 200 feet of 2 inch main supply line, two 75 foot 2-inch distribution line, and an elevation difference between the bottom of the pump and the lateral distribution pipes of 10 feet.

To work effectively, the system should be pressurized at 5 ft of head. The elevation difference from the bottom of the septic tank to the distribution pipe is 10 feet. The total pipe length is 350 feet but friction losses are only calculated in the supply line (not the perforated distribution lines or manifold). Using Table 3, the friction loss per 100 feet of pipe with a flow of 50 gpm is 3.99 ft. Therefore, the pressure loss due to pipe friction is 7.8 feet. (200 ft x 3.99 ft/100 ft). Adding the 25% for additional losses due to joints and elbows the pressure requirements of the pipe is 9.8 feet. The sum of these losses is 21.8 feet (5+10+9.8).

Distribution line slopes

Placement of the dose tank and piping must insure that supply lines and manifold pipes drain back into the dose tank. Final slopes of 1% or more back to the tank are sufficient. To achieve this minimum final slope at all locations in the system, all pipe trenches should be dug at a slopes of 1.5 to 2%. The manifold pipe must be installed below the distribution laterals so the entire system can drain back into the pump tank.

Bark bed covering

Chambers are covered in bark.

Once the distribution system is in place, the infiltration area should be covered with 18 to 24 inches of bark or wood shreds. Currently, there are no design specifications on the bark or wood shreds used. The purpose of the shreds is to insulate the infiltration area, allow good oxygen transfer to the soil, and enhance evaporation. As such, the primary criteria are to have large pore spaces in the material. Breakdown of the wood material over time will reduce the porosity and restrict oxygen transfer to the soil interface thus increasing the risk of soil plugging and seepage from the edge of the bark bed. As such, hardwood bark or wood chips/shreds are preferred to softwoods. Avoid material with lots of small particles or fines.

Avoid driving on the infiltration area during the placement of the wood or bark. This will help maintain soil infiltration capacity.

Inspection and maintenance

Typical layout of a bark bed system using chambers.


A finished bark bed.

Install inspection pipes, vertical capped 4-inch PVC pipe, at the ends of all distribution pipes for inspecting the soil infiltration area. The bottom of the pipe should extend through to the bottom of the rock spreader or through the chamber. This inspection pipe should rise approximately 12-inches above the finished height of the bark or wood and be anchored firmly in place. These inspection pipes allow a visual assessment of ponding in the infiltration area. Excessive ponding is an indication that the soil is plugging due to excessive organic loading to the system or too much water is getting to the system.

Every three months the system should be checked. This monitoring includes a check of the effluent filter, ponding in the infiltration area, and seepage around the perimeter of the bark bed. Excessive buildup of solids on the effluent filter indicate high organic loading. If this occurs, investigate the equipment and management practices in the milk house that might allow excessive waste milk or manure into the system. Seepage around the edges of the bark bed indicates system failure. Seepage could be due to excessive water getting in the treatment system from water leaks in the milk house, soil plugging of the infiltration area due to high organic loading, or channeling of the effluent in the infiltration area. The source of the problem should be investigated and the system repaired.

Add additional bark when depth over the spreader or chamber is less than twelve inches.

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