STUDIES IN AEROBIC DIGESTION OF WASTE ACTIVATED SLUDGE

Abstract

Activated sludge and its modifications are success fully used treatment processes for medium to large size wastewater treatment plants including those used in the industrial waste treatments. However the disposal of waste activated sludge still remains a problem. Aerobic digestion is the newest method for treatment of activated sludge and tried in the endogenous phase of aerobic bacteria. As the supply of available food(substrate) is depleted, microorganisms will begin to consume their own protoplasm to obtain energy for cell maintenance reactions thus achieving endogenous oxidation of cell tissue, in turn stabilising the sludge with reduction in its volume, ulti mately to nonbiodegradable solids. Eventhough it was known as early as 1933 that greater rates of degradation of solids can be achieved under aerobic conditions than anaerobic conditions, the investigators started showing interest in the aerobic digestion from late fifties. Earlier studies were performed in batch aerobic reactors on activated sludges, primary cum activated sludges and biodegradable industrial waste sludges. These studies revealed 10 to 45 percent reduction in volatile suspended solids (VSS) during the process, which reduction increased with increasing retention times (4 hrs to 50-60 days) and digestion temperature (5°C - 30°C). Aerobic digestion studies on continuous sludge flow digesters in the laboratory and iii iv existing plants show about 30-40 percent reduction in vola tile suspended solids and a stable final product. The rate destruction of activated sludge during its endogenous respiration phase can be expressed as a first order function of the concentration of the degradable hiomass present. The kinetic rate constant kd, as determined from batch digestion studies, on the basis of volatile suspended solids reductions, is seen to be used with modification for design of continuous flow completely mixed aerobic digesters. The endogenous decay rate is found to depend on the type of wastewater on which the sludge is grown, sludge concentration, sludge age (the time for which the sludge is aerated prior to digestion) and on environmental factors like pH, digestion temperature, etc. Initial TS concentration upto 25>000 mg/1 was tried for aerobic digestion of activated sludge which showed that the kinetic rate constant decreases with increasing initial TS concentration, however achieving substantial solids reduc tion with 10-15 days aeration time at 20°C. Few investigators reported fixed solids reduction and suggested total suspended solids be considered for design of aerobic digesters. Lower pH values even upto 5.0 (or lower) are experienced during aerobic digestion studies, and these values are reported to be detrimental to the process. This kinetic rate constant k, of endogenous decay is observed to depend on digestion temperature and generally v follows an Arrhenius relationship k. -, • k^ 2o°c e " with 9 varying from 1.02 to 1.07. The relation is established in the mesophilic temperature range (5°C-40°C), and recent investigations show that it is probably also applicable in the thermophilic temperature range (40°C to 60 C) of aerobic digestion. Investigations on endogenous decay in thermophilic range of digestion temperature are encouraging and show that the sludge stabilisation could be achieved in short retention times of less than 5 days at these temperatures, which can be maintained with heat produced during endogenous decay. Present investigations were undertaken to further explore the endogenous decay in a wide range of initial total solids concentration of activated sludge (1,000 mg/1 to 80,000 mg/1), initial pH range of 4.5 to 10.4 (4.5, 6.0, 7.7, 9.0 and 10.4), digestion temperature range of 5°C to 60°C (5°, 10°, 20°, 30°, 35°, 40°, 45°, 50° and 60°) and to study the occurrence of nitrification and its progress during aerobic digestion. Activated sludges of desired total solids concentra tions were grown in the laboratory by aerating domestic sewage enriched with organic solids from human excretal materials, followed by the sludge thickening when necessary. The laboratory batch digestion reactors were of 5 1 capa city and the aeration was achieved by bubbling air through circular nylon pressure pipe distributors located at the bottom of each reactor which maintained the sludge dissolved vi oxygen (DO) at a level around 2 to 3 mg/1. The digestion temperature was maintained at the desired level + 1°C by keeping the reactors in thermostatically controlled water baths. The quality of the sludges undergoing aerobic diges tion was monitored through representative samples collected from reactor units at desired time interval and tested in the laboratory for total solids (TS), volatile solids (VS), fixed solids (FS), 5-days biochemical oxygen demand (BOD), chemical oxygen demand (COD), alkalinity, pH, organic nitrogen, ammonia nitrogen, nitrite nitrogen and nitrate nitrogen. A concept of "digestion period" is introduced and defined as the aeration time required for near completion of aerobic digestion of activated sludge. During the data analysis of each experimental run the digestion of sludge was presumed to be completed when variables TS, VS ,BOD and COD versus aeration time became asymptotic to x-axis and this time of aeration required for completion of digestion is referred as "digestion period". The rate of endogenous decay kd (kinetic rate coefficient, to the base e) when determined on the basis of reductions in VS, TS, BOD and COD is respec tively referred to as k^g, k™, k^ and kc. The findings of the data analysis of the experimental runs conducted under controlled conditions are narrated below ~ vii 1. Fixed solids concentration remained almost constant during entire period of aerobic digestion; and progressive reductions in TS/VS concentrations and BOD/COD values were essentially due to progressive reductions in activated sludge biomass. Biodegradable part of these variables formed a fixed percentage of their respective initial values. It is seen that BOD/COD values of the activated sludge can be used for expressing progress of aerobic digestion and its kinetics. 2. Digestion period is observed to increase with initial solids concentration or initial BOD/COD values of activated sludges undergoing aerobic digestion; however remained at almost same values for activated sludges having around same initial concentrations of these variables, irrespective of the different digestion temperatures. The percentage reduc tions in these variables (at the end of the digestion period) were greatly affected by the temperatures increasing with digestion temperature in 5°C to 35°C range. The analysis of data offers predictive models for digestion period and progress of aerobic digestion in 5 to 35 C digestion temperature range. 3. Initial pH of around 8.0 was the optimum pH value yielding minimum digestion time and maximum kinetic rate constant. 4. The kinetic rate constants were observed to decrease with increasing concentrations of variables, and to follow VI11 exponential relationship with initial concentrations of these variables. 5. The kinetic rate constants increased with digestion temperature in 5° to 35°C range. The kyg and k^ followed an Arrhenius relationship kd#T» kd.20,e ' with QhavinS avalue of 1.056 and ^d%20^VS^S value at 20°C) of 0.20 -1 day . 6. The observations of the occurrence and progress of nitrification phases (nitrite formation followed by nitrate formation) during the present investigations lead to the following conclusions: (a) Favourable ranges of temperature and pH for nitrifica tion were respectively observed to be 25 to 30 C and 6.0 to 8.3. The system pH was seen to reduce to 6.0 or even lower during the nitrite formation phase of nitrification, and these reduced pH values were det rimental to nitrification. (b) Almost complete elimination of activated sludge bio mass was one of the essential pre-requisites to initiate nitrification during aerobic digestion of activated sludge. (c) With all favourable conditions a minimum period of about 2 days was necessary for population built up of genera Nitrosomonas and Nitrobacter and in turn to start asserting themselves and initiate nitrification during aerobic digestion. IX (d) Nitrate formation invariably lagged behind nitrite formation . However with all favourable environmental conditions and sufficient population built up of nitrifying bacteria both phases of nitrification were observed to progress hand in hand. (e) Rate of nitrite formation was invariably more than that of nitrate formation under similar environmental conditions and indicates that genus Nitrosomonas grow faster than genus Nitrobacter (which are responsible for nitrite and nitrate formations respectively).