Small Community And On-Site Land Application Systems -
Design And Construction

Abstract - Rob Potts

The design of on-site and small community wastewater land application systems has historically been based on soil hydraulics, with an allowance for a long-term reduction due to biological slimes, and more recently an allowable increase in loading if the effluent is of higher quality. This is not considered land treatment but land disposal as there is little allowance for renovation of the contaminants apart from biochemical oxygen demand (BOD), suspended solids, and perhaps microbiological. As the ultimate receiving environment is generally groundwater and then perhaps surface water, the design needs to focus on the soil, plant and groundwater ability to assimilate other contaminants and nutrients in respect to the sensitivity of the receiving environment.

There are two interrelated parts to on-site and small community systems - the treatment component and the discharge component. Ultimately it is the discharge component that determines the design of the both components of the treatment system. So much focus tends to be placed on the treatment unit with still a poor understanding of the land treatment component. Design of land application systems, particularly on-site systems, is generally too generic and based on cookbook type guidelines. Although these provide good general design principals, they are taken as “The Bible” with no room for engineering judgement or thinking beyond the square. This is a fault of both designers and consent authorities, and often indicates a limited understanding of the discharge and treatment system. Understanding the soil and physical processes that are occurring to make the soil what it is are more important than undertaking a quick snap shot of the soil to determine surface profile conditions or doing percolation tests. By all means the profile has to be assessed and to a lesser extent percolation rates determined, but understanding the reasons why the soil has formed and identifying limitations to land treatment are of far greater importance. It is more cost effective for the client to use a digger for an hour to go down 2 – 3 m in four to six locations, than for the designer to hand auger down 1 – 1.5 m and not get the whole soil and subsoil picture.

Are we getting the design right – compared to overseas data, we are over designing (under loading) but systems are still failing in some areas – why? Is it the discharge system, too much reliance on the black box treatment unit or is it how the system is managed?

The discussion will briefly cover design parameters and issues for a number of land application systems and their components, such as filtration, LPED trenches, drippers, mini-sprinklers, wetlands, and overland flow. Where possible, traps for young players and cost effective alterations to standard specifications will be outlined – however, some of these will require buy in from Council Regulatory Staff. It will conclude with a couple of examples, some questions to ponder and thoughts to hopefully provoke discussion.