One approach to reduce nutrient losses from livestock farms is to apply biological waste treatment systems such as biological nitrogen (N) removal or enhanced biological phosphorus (P) removal (EBPR) to reduce the nutrient content of land-applied waste. The EBPR process takes advantage of the ability of P-accumulating organisms (PAOs) to sequester excess P as polyphosphate granules in their cytoplasms, yielding a P-depleted liquid effluent and a P-enriched biomass. Biological N removal systems result in the conversion of organic or ammonia-N to innocuous N-2 gas. Understanding the variation in parameters such as chemical oxygen demand (COD), total and volatile suspended solids (TSS and VSS), and ammonia-N (NH3-N) is necessary to design these systems. Our objectives were to evaluate the effects of diet and manure separation on parameters important to reactor design. Waste was collected from nine cows fed a high P diet (0.47% P), a low P diet (0.32 % P), or low P with exogenous phytase plus cellulase (0.32 % P), in a replicated Latin square design (three 3 X 3 squares). Total collection of milk, urine, and feces was conducted on days 19 to 21 of each period, a mixed slurry (urine, feces, and water) was created, and slurry was separated mechanically to generate liquid effluent. Slurry contained more COD, solids, N, and P than liquid effluent, but the COD:P ratio was similar in the two wastes. The ratio of COD:N was higher in slurry than in separator effluent, but the ratio in both wastes was sufficient to support biological N removal. The P content of slurry, liquid effluent, and manure solids from cows fed low P was lower than from cows fed high P, and the COD content of effluent was higher with the low P diet. The COD:P ratio of all wastes was sufficient to support EBPR and biological N removal, but variation was observed with diet. Waste from cows fed low P had a higher COD:P ratio than that of cows fed high P, and waste from cows fed the enzyme-supplemented diet had a lower COD:N ration than that of cows fed the control diet. Dairy manure slurry and effluent will support EBPR and biological N removal. Dietary effects on parameters important to the design of advanced waste treatment systems were observed, but were not of a magnitude that would affect reactor design.