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DEVELOPMENT OF A RATIONAL DESIGN PROCEDURE FOR OVERLAND FLOW SYSTEMS C.J. Martel, T.F. Jenkins, C.J. Diener and P.L. Butler INTRODUCTION Background An overland flow system consists of a series of grassy terraces which are carefully graded so that L wastewater flows downslope in a thin sheet. Gated pipe, slotted troughs, or sprinklers are used to uni- r q ' formly distribute the wastewater at the top of each terrace,and a ditch or channel collects the runoff at the base of the terrace (Fig. 1). When properly s designed and managed, the quality of runoff from an overland flow system can easily meet secondary effluent standards. Unlike most wastewater treatment systems, overland flow reduces the volume of water to be _. discharged. This reduction in volume is caused by Figure 1. Concept drawing of overland flow system losses through evapotranspiration and percolation (where L is terrace length, q average overland flow as wastewater flows over the soil surface. Typically, rate and s the slope), the volume of runoff is 60 to 90%of that applied. Consequently, removal efficiency should be calcu- an economical return in the form of high quality lated on a mass rather than concentration basis. forage crops. Palazzo et al. (in prep.)estimated the Overland flow systems offer a number of ad- value of this crop to be $858 ha' per year. vantages over conventional treatment. First and In spite of these advantages,not many overland foremost is the lower cost of operation and main- flow systems have been built. One of the main tenance. Highly skilled personnel are not needed reasons is the lack of a rational procedure for design. to run the facility and energy requirements are The procedure presented in U.S. EPA (1977) is based significantly lower (Middlebrooks and Middlebrooks on general guidelines which are difficult for inexper- 1979). Overland flow systems are also very reliable ienced engineers to interpret. For example,when and able to withstand large variations in strength untreated or primary effluent is applied, the designer of applied wastewater without system upset (Aly is advised to select a hydraulic loading rate within a et al. 1979). Another important advantage is that range of 6.4 to 20 cm wk-1,depending on climate, no sludge is produced except by pretreatment degree of treatment and detention time. Without processes. Overland flow systems can also provide previous experience it is difficult for the designer to