Introduction. Sludge is a broad term used to describe the various aqueous suspensions of solids produced during wastewater treatment. Raw sludge can be broadly classified as primary sludge, secondary sludge and mixed sludge (mixtures of these two sludge). Raw sludge contains a variety of pollutants creating potentials for environmental pollution and must be treated and disposed properly. Sludge produced in Isfahan South Wastewater Treatment Plant is treated by anaerobic digestion. This method generates nuisance odors and has many operational problems.
Aerobic digestion is an alternative process for stabilizing sludge. Because the operational control of aerobic digesters is simple and many other advantages, it appears that aerobic digestion will increase in popularity.
Methods. Aerobic digestion was carried out in a 7.5 liters bioreactor (rectangular glass tank 23*16*21 Cm) that was loaded with 5 liters mixed sludge from this plant. This experimental research was conducted at the Environmental Health Laboratory of Isfahan Faculty of Health. Aeration of bioreactor content provided through three air stones that each of them connected with one aquarium air pump (RENA101). The aeration rate was set to maintain a minimum residual dissolved oxygen concentration of 1.5 mg/I and aeration was sufficient to keep the solids in suspension. This research was continued for 5 period from autumn and winter 1998 through end of spring 1999. In each period the sludge was aerated for three weeks and in each week two times samplings are accomplished and were analyzed.
Results. The measurements of TS, TSS, TVS, VSS, and COD concentration and percent reduction of them were performed to monitor the progress of aerobic digestion process and determination of design parameters. Standard Methods (APHA, 1992) were used for all analytical procedures. After the 18 days of detention time of aerobic digestion the decrease in TS, TSS, TVS. VSS, and COD reached 45.22, 49.30, 52.36, 52.28, and 43.03 percent respectively of its primary value.
Discussion. The results of this study were shown that mixed sludge from Isfahan South Wastewater Treatment Plant were stabilized effectively by aerobic digestion and during of these process significant decreases in any form solids and organic matters were occurred. Determining endpoints in digestion of sludge is carried out by determining the TVS reduction. A 40 percent TVS reduction (EPA's regulations) was achieved with a minimum hydraulic detention time of 11 days at maximum TVS loading rate of 1.45KgTVsm-3d-1.