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by the same rate-limiting step. It is unlikely that Phosphorus removal can be improved by adding both substrates would have the same removal rate alum to the wastewater prior to application. Thomas constant;a more likely explanation is that removal et al. (1976) increased phosphorus removal to 900/g rate is mass transport limited. In other words, the using this technique. Similar results were obtained rate of mass transport from the bulk liquid to the at the Utica, Mississippi,overland flow site (Peters active biomass and adsorption sites is the mechanism et al. 1981). governing removal rate. This reasoning is reinforced by the fact that overland flow operates in a laminar Validation flow regime, which reduces the opportunities for The kinetic relationships for removal of BOD, substrate contact with reactive sites. TSS and NH3-N were validated by comparing the predicted removal to the actual removal reported Phosphorus removal at seven full-scale systems. Statistical analysis of According.to the Process Design Manual for Land these data indicated that the average differences Treatment of Municipal Wastewater (U.S. EPA 1977) between predicted and actual BOD,TSS and NH3-N phosphorus is removed primarily by sorption to soil removal were only 1.9, -2.0 and 2.8%,respectively, particles. On overland flow terraces only surface for systems receiving primary or raw wastewater exchange sites are available because most of the (see Table 4). However, when systems receiving wastewater passes over the soil surface rather than pond effluent were evaluated, the predicted removals through it. As a result, the exchange sites are used for BOD and TSS were 18 and 22%higher than up rather quickly and the removal of phosphorus by actual (see Table 5). Higher predicted removals can overland flow systems is limited. Plant uptake is be explained by the fact that pollutants remaining another mechanism capable of removing phosphorus. in pond effluent are generally less degradable,and Palazzo et al. (1980) reported that forage grasses thus more difficult to remove, than those in primary removed 54%of the applied phosphorus at the or raw wastewater. Also, there is a lower limit to the CRREL site. BOD and T55 concentration in the runoff. As dis- t As shown in Figure 10, our studies indicated cussed earlier, this limit is approximately 5.0 mg L`.t. that phosphorus removal did not change significantly Therefore, high removal efficiencies become more over the range of detention times tested. Percentage difficult to achieve as pollutant concentrations in removals ranged between 37 and 61%and averaged the applied wastewater decrease. 53%. Analyses of runoff samples indicated that The ammonia removal relationship (Fig.9) appears most of the total phosphorus was in the "ortho" to be valid whether primary or pond effluent is form, which indicates that the phosphorus removed applied. The average differences between predicted I' was tied up with particulate matter. As discussed and actual NH3-N removal were only 2.8 and-4.5% earlier (see TSS Removal), particulate matter was for systems receiving primary and pond effluent, easily removed by overland flow. respectively. loo d. 80 60 a• ' E u40— p n Average Removal: 53% 0 r 20 (o) Section A I (•) Section B (�) SeU ion C I O 20 40 60 80 f, Average Detention Time (min) Figure 10. Total phosphorus removal vs detention time. 11