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terraces,each.45 m long x 4.5 m wide and slopes Temperature also has an effect on runoff BOD of 2, 4 and 8% (Peters et al. 1981). Detention times concentrations. Martel et al. (1980) found that times were measured using the same procedure BOD concentrations in the runoff exceeded 30 mg/L developed at CRREL. �atsoll temperatures at or below 4°C. However, The second site is located indoors at the Univer- temperature effects shou not e a significant sity of California at Davis. Each laboratory scale problem at full-scale facilities if wastewater is model terrace is 6 m long x 1.5 m wide and set at stored during the winter. In this study, temperature a 4%slope (Smith et al. 1980). Deionized water effects were nullified by selecting performance data was used as the tracer and a response curve was obtained during the growing season only (April developed by measuring specific conductance. through October). A combined total of 40 detention time measure- The experimental data obtained at CRREL and ments were taken at both sites. Measured detention the University of California, Davis (Fig. 6) indicate times are shown in Table 3 along with the predicted that BOD removal can be expressed as a first-order detention times calculated from eq 13. The average equation in the form difference between predicted and measured deten- tion times was only 8 minutes. In most cases the measured detention time was longer than predicted, Percent removal= (1 -A e kT) 100. (14) which allows an extra margin of safety in the design. The coefficients and k,obtained by a least- In a Student's t distribution, the difference between I measured and predicted detention time was not squares fit to the data,were 0.52 and 0.03 min { significant at the 95% level. respectively. The coefficient k is the average kinetic Although the average difference between pre- rate constant. The coefficientA can be interpreted dicted and measured detention time was not signifi- as the non-settleable fraction of BOD in the applied cant, individual differences were considerable. This wastewater while the remaining settleable fraction is understandable, considering the variability of the (0.48) is removed during the first few meters or surface microtopography from one terrace to another. minutes after wastewater is applied. This conclusion Construction techniques, patterns of vegetative is supported by the BOD vs downslope distance data growth and harvesting operations are also factors shown in Figure 7 where 44%of the BOD was re- which can change the hydraulic detention time. moved ved within the first 5 m. TSS removal KINETICS Total suspended solid (TSS) removal v�ragee detention time from the CRREL site is shown in Kinetic relationships describing removal of BOD, ig�ure .Thflat slope of the estimated line of TSS, NH3-N and total P were developed by taking best fit indicates that TSS removal changed little several detention time measurements during each over the range of detention times tested.( oma r example, application period. The average detention time at a detention time of 20 minutes,TSS removal was (T)was then calculated along with the average per- .....86%. A three-fold increase in detention time (60 cent removal on a mass basis for each constituent. min) only increased removal by 6%. All raw data used in this development can be found The high solids removal efficiency of the overland i in Appendix A. flow process is due to the shallow depth of water and the long travel distance to the end of the terrace. BOD removal Even minute particles with slow settling velocities BOD is removed by sedimentation,filtration and are able to settle out before reaching the collection biological oxidation (U.S. EPA 1977). The first ditch. Also, grass and vegetative litter help to entrap two mechanisms are responsible for removing par- and filter out particles. Data plotted in Figure 7 ticulate BOD. The soluble BOD is oxidized by mi- indicate that most of the suspended solids were re- croorganisms which are probably similar to the moved within the first 5 m. attached biomass found in trickling filters. However, Because of rapid settling,a buildup of solids is some soluble organic compounds are released from apparent at the top of most overland flow terraces the plant-soil system,and as a result,runoff ROD which receive raw or primary wastewater. At the concentrations below to 5 m L cannot be ex- site, so sition was eav s epoy enough in petted (Overcash et al. 1976). some spots to smother grass growth. Similar condi- 7