In developing economies such as Morocco, nearly 70% of all of the wastewater treatment plants are not functioning due to lack of spare parts and poor cost recovery (USEPA, September 2004). Intermittent and recirculating sand filters offer a treatment technology with minimal electromechanical
components and low operating costs, which is advantageous to sustainable wastewater treatment for developing economies. When properly designed with the correct nitrification kinetics and operated to prevent accumulation of excess biofilm biomass within the filters, such systems can achieve
a high degree of nitrification. The use of recirculating sand filters in conjunction with low-maintenance, low-cost technologies provide an affordable treatment system for nitrogen removal with beneficial opportunities. Chemical Oxygen Demand (COD) removal and nitrification rates were determined
from converting unaerated recirculating sand filter (U-RSF) to an aerated recirculating sand filter (A-RSF) for full-scale installations in the U.S. and abroad. These rates were adapted to design U-RSFs for carbon removal and nitrification at additional wastewater treatment facilities in developing
economies. The COD removal and nitrification rates achieved in an U-RSF filter which was overloaded were successfully increased with aeration to reduce effluent ammonium-N levels from a range of 10 to 20 mg/L to less than 1 mg/L. The differences in COD removal and nitrification kinetics
observed during unaerated and aerated conditions were studied and applied to the design of U-RSFs for nitrogen removal in Morocco and Jordan. Upflow biological aerated filters (BAFs) with periodic backwashing, such as Infilco Degremont's Biofor® and Kruger's Biostyr®,
can achieve simultaneous carbon removal and nitrification, with rates of 1 kg/m3/d at liquid temperatures of 12 to 15°C, when the influent soluble biodegradable COD is less than 20 mg/L. These conditions typically exist downstream of a first-stage COD removal system,
such as an activated sludge or fixed-film biological treatment system. The nitrification rate achieved in an A-RSF operating without backwash was lower, averaging 0.5 kg/m3/d at temperatures of 10 to 227°C. For U-RSFs installed without aeration systems in developing
economies, the only means of aerating the biofilm is by raking the filter surface and complete gravity drainage between filter loadings. The rate observed for U-RSFs with this type of operating strategy was 0.25 kg/m3/d. This nitrification rate for U-RSFs has been used successfully
used to design wastewater treatment facilities in Morocco and Jordan. The researchers have monitored the consistent nitrification and d performance of these U-RSFs for several years and concluded that: 1. Simple and reliable biological
nitrogen removal systems using biofilm processes, such as the U-RSF, can be designed for municipal wastewater treatment systems in developing economies. U-RSFs are an effective treatment technology that satisfy the economic constraints often faced by small communities. A
biofilm system is a comparable substitute for the activated sludge treatment process. The biofilm process requires less operator intervention and maintenance. Pre-anoxic lagoons can be combined with nitrate recycle from a U-RSF to design reliable BNR
systems for developing economies. Nitrification rates of 0.25 kg/m3/d may be used for the design of U-RSF in which the DO levels are less than 0.5 to 1 mg/L.
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