Guidelines for the Design and Construction of Sub-Surface Irrigation

Transcription

1 Guidelines for the Design and Construction of Sub-Surface Irrigation

2 Contents How it Works. Page 4 Sub-Surface Irrigation.. Page 4 What Not To Do. Page 4 Designing the Treated Wastewater System. Page 4 Dispersal Rates. Page 5 Daily Wastewater Flow Rates. Page 6 How to Calculate Dispersal Area Page 7 Irrigation Area in m.. Page 7 Appendix 1: Minimum Setback Distances Required for Wastewater Disposal Areas.. Page 8

3 Guidelines for the Design and Construction of Sub-Surface Irrigation How it Works Sub-surface irrigation provides the most sustainable use of recycled water because the water is applied directly to the plant roots. By dispersing the treated wastewater into soil at a very slow rate, the subsurface soil serves as a large filter. The vegetation growth on top of the soil and micro organisms within the soil remove organic matter and nutrients therefore reduce the amount of nutrient and bacterial contamination leaching into the groundwater. Sub-Surface Irrigation The default land application system for sustainably recycling secondary treated sewage or greywater effluent to land is pressure-compensating sub-surface irrigation (with disc or mesh filters and scour and vacuum valves) installed along the contour, which evenly distributes effluent throughout the irrigation area. Using drip-line with uniformly spaced drippers/emitters makes it possible to apply and disperse treated wastewater evenly over the entire dispersal area, causing even absorption and distribution. The distribution pipes (drip-lines) fill up with effluent until a certain pressure is reached which opens the emitter valves. For a 450m² irrigation field with 1mm diameter pipes, at least 60L may be required to be pumped into the pipes to reach the required pressure to open the emitters. More controlled pressure can be applied when the field is divided into two or more zones and these smaller areas are intermittently dosed using a sequencing valve. What NOT to do: A gravity-flow effluent irrigation system is not allowed, due to the lack of even distribution. Irrigation distribution pipes must not have dripper-holes drilled or cut into them after purchase because the effluent will flow out of the holes in the first few metres of pipe at a far higher rate than the system is designed for and higher than the soild is capable of sustainably absorbing. Designing the Treated Wastewater Dispersal System When designing a dispersal system for treated wastewater, the main parameters affecting the design and size of the systems are as follows: The amount of treated water to be dispersed, The dispersal rate (hydraulic loading rate) to the soil. (see table 1 for recommended dispersal rates) The total area available for the dispersal system Council requirements. Other factors that must be considered include slope and landscape contours, vegetation cover, watercourses and setback distances (Appendix 1). Consideration must be given to nutrient balance for land application in environmentally sensitive areas.

4 Dispersal Rates The acceptable dispersal rate depends on various factors as discussed. The values in the following table represent a conservative approach for sizing a dispersal system taking all parameters into account. Table 1 Soil Category Soil Texture Structure 1 Gravels and sand Structureless Massive Sandy loams Indicative permeability (K sat ) (md) (see Note 1) Design irrigation rate (DIR) (mm/day) (see Note ) Indicative drainage class (See Note ) >.0 5 Rapidly drained >.0 5 Well drained Massive Loams High/moderately Moderately well drained or massive Clay loams High/moderately Imperfectly drained Massive Light clays Strongly Poorly drained Moderately or massive < Medium to heavy clays Strongly Very poorly drained Moderately <0.06 or massive <0.06 NOTES: 1. The values of indicative permeability as K sat are based on the movement of water, and not effluent through the soil. They are estimates only and should be used with caution in determining soil category and Design Loading Rates.. The Design Irrigation Rates are based on EPA Code of Practice Indicative drainage classes listed are based on the assumption that drainage of water out of the soil is governed only by the indicative permeability and that external factors play no role. 4

5 Daily Wastewater Flow Rates Daily household wastewater generation is estimated by multiplying the potential occupancy, which is based on the number of bedroom (plus one person) by the Minimum Wastewater Flow Rates (Table 4 EPA Code of Practice 891.). Table Source Design hydraulic flow rates for all water supplies (L/ person.day) Organic material loading design rates (g BOD/person.day) Households with extra wastewater producing facilities (eg. spa baths) 0 60 Households with standard water fixtures Households with full water-reduction fixtures (min. 4 Stars for dual-flush toilets, showerflow restrictors, aerator taps, flow/pressure control valves and min. Stars for all appliances, eg water-conserving automatic clothes washing machines) Any additional room(s) shown on the house plan such as a study, library or sunroom that could be closed off with a door, should be considered a bedroom of the purposes of the calculations. Example: A. Water-reduction fixtures For a 4 bedroom house with water reduction fixtures and fittings, the daily minimum wastewater generated is: Daily Wastewater Volume = (Number of bedrooms +1) x L/person.day B. Standard water fixtures = (4 + 1) x 150 = 5 x 150 = 750 L/day For a 4 bedroom house with water reduction fixtures and fittings, the daily minimum wastewater generated is: Daily Wastewater Volume = (Number of bedrooms +1) x L/person.day = (4 +1) x 180 = 5 x 180 = 900 L/day 5

6 How to Calculate Dispersal Area The dispersal area can be calculated by using the daily household wastewater flow and the Design Irrigation Rate (DIR). A = Q / DIR Where Q = Daily household wastewater flow (Table ) And DIR = Design Irrigation Rate (Table 1) Example: The dispersal area required for a 4 bedroom house with water reduction fixtures in medium clay soil can be calculated by: Q = (4 + 1) x 150 (Table ) = 750 DIR = (Medium Clay Soil DIR from Table 1) Therefore: A = 750 / = 50 m of subsurface irrigation area required. Irrigation Area in m To assist in the design of the surface or shallow sub-surface irrigation area, the following table recommend minimum wastewater disposal field sizing. In the absence of additional information and recommendations provided in the form of a Land Capability Assessment and specific soil analysis, the figures in the table are the minimum area required based on permeability rates for the poorest quality soils in the area. to ensure a conservative approach. A good quality topsoil may still be required for highly dispersive soils using sub-surface irrigation. Number of bedrooms Minimum Disposal Area Required (m ) 1 bedroom dwelling 140 bedroom dwelling 10 bedroom dwelling 80 4 bedroom dwelling 50 5 bedroom dwelling 40 6 bedroom dwelling bedroom dwelling 560 The figures provided above are based on the EPA Code of Practice 891., an indicative soil permeability rate of 0.06 m/day (soil rating 6) and a daily wastewater flow rate of 150 L/person/day + 1. For Further Information Please Contact Warrnambool City Council s Environmental Health Team Phone: Fax: Mail: PO BOX 198 Warrnambool, VIC 80 Website: 6

7 Appendix 1: Minimum Setback Requirements for Wastewater Disposal Areas LANDSCAPE FEATURE OR STRUCTURE BUILDING Septic Tank SETBACK DISTANC- Onsite Wastewater Treatment Plant 1 Advanced secondary greywater effluent Wastewater field up-slope of building 6 Wastewater field down-slope of building Wastewater up-slope of cutting/escarpment ALLOTMENT BOUNDARY Wastewater field up-slope of adjacent Lot 6 1 Wastewater field down-slope of adjacent Lot SERVICES Water supply pipe Wastewater up-slope of potable supply channel Wastewater field down-slope of potable supply channel Gas supply pipe In-ground water tank 15 4 Storm water drain 6 RECREATIONAL AREAS Children s grassed playground 6 In-ground swimming pool 6 SURFACE WATER (UP-SLOPE FROM) Dam, lake or reservoir (potable, including food production)* Waterways (potable water supply) Waterways, wetlands (continuous or ephemeral, nonpotable); estuaries, ocean beach at high-tide, dams, lake or reservoirs (stock and domestic, non-potable) Waterways (potable Water Supply) Drainage lines, dam outfalls GROUNDWATER BORE Category 1 and a soils NA Category b to 6 soils WATERTABLE Vertical depth from base of trench to the highest seasonal Vertical depth from irrigation pipes to the highest seasonal NA The setback distances may be reduced by up to 50% if the effluent is treated by a secondary wastewater treatment plant that is capable of producing effluent that complies with the 0/0/10 standard and that the effluent is dispersed by sub surface irrigation.. To applicant must provide verification that the treatment plant will be maintained in accordance with the EPA Certificate of Approval conditions.. Setback distances may be increased if the slope or gradient of wastewater envelope is greater than 5%. 7