Anaerobic reactor development for complex organic wastewater

Transcription

1 Anaerobic reactor development for complex organic wastewater Synopsis Anaerobic treatment is applied extensively of removal of organic pollutants (COD) from wastewater. It is more competitive than aerobic treatment in applications where the quantity of COD to be removed is large. The major fraction of COD is converted to useful methane gas, and only a small fraction becomes waste sludge. The COD loading rate of anaerobic reactors is higher than that of aerobic reactors and hence small reactors are sufficient for the treatment of same quantity of COD. Chapter 2 surveys anaerobic technology and identifies directions for improving reactor technology. High-rate anaerobic reactors have reduced the cost of anaerobic treatment plants. High-rate reactors routinely achieve organic loading rates 8 to 10 kg COD/ m 3 reactor / d and hydraulic retention times of the order of a few hours. High-rate reactor designs can be broadly classified as fixed film (eg. fixed film, fluidized bed) and suspended growth reactors (eg. UASB, EGSB). The fixed film reactors provide an inert carrier media for growth of anaerobic consortia as a biofilm. Biomass is retained as settleable flocs or granules in suspended growth reactors. The UASB is the most common reactor in use. The CSTR is a suspended growth reactor but is not a high-rate reactor since there is no mechanism to separate and retain biomass. High-rate reactors have been successfully applied for the treatment of a wide range of industrial and domestic wastewater. However, wastewaters containing degradable COD in mostly particulate form, is not treatable at high-rate in these reactors. Such effluents are termed complex organic wastewater in this thesis. Examples of such wastewaters include dairy effluent, slaughterhouse effluent and palm oil mill effluent. Municipal sewage can also be considered as complex organic wastewater. The development of a high-rate anaerobic reactor capable of treatment of complex wastewater is necessary. This thesis concerns the development of a new high-rate anaerobic reactor called the Buoyant Filter Bioreactor BFBR for the high-rate treatment of complex organic wastewater. Current high-rate anaerobic reactors are based on the principle of decoupling biomass retention times from the hydraulic retention times. This works only when the rate limiting step in the reactor is a microbial growth process typically acetoclastic

2 methanogenesis. In the treatment of complex wastewater, the rate limiting step is exocellular enzymatic hydrolysis. The central hypothesis in the development of the BFBR is that high-rate treatment of complex wastewater requires the decoupling of particulate- COD retention time from the hydraulic retention time. Particulate COD, if retained sufficiently long in the reactor should undergo complete conversion. The BFBR is designed to retain particulates with the reactor using a deep-bed filter system. Chapter 4 describes the development of BFBR. The BFBR has an upper chamber and a lower chamber. Between the two chamber is a buoyant filter bed that filters the reactor liquor. The filter media is made from expanded polystyrene beads. The feed wastewater is pumped into the lower chamber which contains methanogenic sludge. Gas produced accumulates in the lower chamber, while the liquor filters through the buoyant filter bed into the upper chamber from where it can overflow. The gas accumulated in the lower chamber is released periodically. During gas release, filtered liquor from the upper chamber flow back through the filter bed into the lower chamber, fluidizing the filter bed in the downward direction. The solids captured in the filter bed are backwashed into the lower chamber. The periodic gas release is achieved automatically using a gas siphon system. Chapter 5 gives the materials and methods used to study the performance of the BFBR. The fabrication of the BFBR and the methods of testing and monitoring performance are described. The BFBR was operated with complex wastewater prepared from full fat milk. All nutrients were provided in sufficient quantity. Another effluent was prepared with oleate emulsion as the sole carbon source. Chapter 6 gives results of the experiments. Prior to reactor operation, the buoyant filter was characterised by filtration tests on bulking anaerobic digester sludge. At filtration velocity 1 m/h, it was found that filter efficiencies were in the range of 70 % for 1 to 2 mm filter media, and 90% for 0.5 to 1 mm filter media. The pressure drop build up was linearly related to filtration velocity. At filtration velocity up to 1 m/h, the pressure drop for 1 to 2 mm filter media reached 10 cm H 2 O in about 15 minutes for 1 to 2 mm media, and in 5 minutes for 0.5 to 1mm media. Operating the filter at higher pressure deforms the EPS bead media and causes non-linear increase in pressure drop.

3 The fluidization velocity for backwash was determined and found to follow the Richarson-Zaki formula quite well. The filter bed is effectively cleaned by backwashing. But if the filter is operated at high pressure, sludge and filter media bond to form aggregates that are not broken up during backwash. The BFBR was operated for more than 400 d with milk effluent. COD loads up to 8 kg/m 2 /d were applied. There was no choking of the filter bed in long term operation. The filter backwash by fluidization was applied at 15 to 20 minute intervals automatically. The filter pressure drop during operation never exceeded 15 cm H 2 O. The COD removal efficiency at steady state was above 85% at all the OLRs applied. The maximum organic loading rate applied during the period reported is 10 kg COD/(m3.day). COD removal efficiency during steady state at this loading was 90%. Through out the operation of the BFBR, effluent COD was less than 450 mg/l. During pseudo-steady state at all loading rates, the effluent COD was less than 250 mg/l. On prolonged steady operation, effluent quality improved and very low COD was obtained even at high organic loading rates. Towards the end of the reported period, with feed COD was in the range of 3200 to 3500 mg/l, the effluent total COD was only 120 mg/l total of which soluble COD was 80 mg/l. Unexpectedly, the BFBR sludge started showing good settleability, with irregular shaped dense flocs. Microscopic examination showed the presence of protozoa in the sludge. These are anaerobic protozoa and are capable of ingesting particulates. The protozoa contain endosymbiontic methanogens that presumably convert hydrogen and acetate to methane. The population of protozoa in the BFBR shows a succession from small rounds to amoeboids to flagellates to ciliates. The residual COD in the BFBR effluent is negatively correlated to ciliate numbers in the sludge. The ciliate rich BFBR treated effluent is very clear, reminiscent of activated sludge treatment. Chapter 7 develops a simulation model for the BFBR in order to get more insight into the process dynamics of the system. The idealized BFBR is represented as a CSTR with a zero volume filter that has specified efficiency for retention of each particulate component. The reactor model is combined with an anaerobic process model, similar to ADM1. The rate processes are taken as microbial growth, decay, enzymatic hydrolysis and gas transfer. The process model has 8 soluble components, 14 particulate components, 4 soluble inorganic components and 3 gas components. The number of processes considered is 25. The model is implemented in MATLAB and has been designed

4 to insert new components and processes without reprogramming. The model has careful accounting of COD, total carbon, nitrogen, sulphur and charge balances. ph is estimated from by solving algebraic charge balance equation at each time step. The model was used to simulate the performance of BFBR with sewage and with milk effluent. The expected performance is obtained by adjusting the filtration efficiency parameters. It is seen that retention efficiency for microbial biomass exceeds the filtration efficiency measured during filtration study with bulking sludge. This implies that mechanisms that improve filterability, such as biologically induced flocculation or granulation are responsible for the retention of the required mass of bacteria. On the other hand, particulate substrates are retained by physical filtration mechanism. Therefore, we conclude that although growth rate of microorganisms such as acetoclastic methanogens is slower than hydrolysis of particulates, the microbial substrate uptake rates in a reactor are higher than particulate hydrolysis rates because organisms accumulate, while particulates get washed out. Hence active methods of retaining particulates inside the reactor, as in the BFBR, increase the overall reactor COD loading and conversion rate. The model reproduces the behaviour of BFBR with a initial build up in concentration of particulates in the reactor followed by degradation during start-up. The model also shows that BFBR is suitable for sewage treatment. Chapter 8 discusses aspects of scale-up of BFBR for field application. The aspects discussed are: constraints on the reactor vessel because of the arrangement of filter, gas accumulator and filtered effluent storage are bought out. The gas-solids-separators optimization for BFBR. Selection of mixing system Design of automatic gas release system The final chapter gives the conclusions and a comparision of BFBR with other anaerobic reactors and a discussion of aspects of scale-up for future development of the BFBR. A summary of the operating parameters of the BFBR is given below: The recommended process design parameters for the BFBR are summarised below: a. Organic loading rate for complex organic wastewater COD loading rate: 6 to 8 kg COD /m 3 /d. b. Filter specifications: Filter media size: 1 to 1.5 mm

5 Filter depth: 10 to 15 cm Filtration velocity: 1 to 2 m/h Filter pressure drop: < 20 cm w.c. Filter backwash velocity: 130 m/h Bed expansion: 30% Filter backwash interval: 15 to 30 minutes Filter backwash volume: > 100% of filter volume References: 1. US 6,592,751 dated July 15, Device for treatment of wastewater. Inventor: Ajit Haridas; Assignee. Council of Scientific and Industrial Research, New Delhi. 2. Ajit Haridas, S. Suresh, K.R. Chitra, V.B. Manilal The Buoyant Filter Bioreactor: a high-rate anaerobic reactor for complex wastewater process dynamics with dairy effluent. Water Research Priya M, Haridas A, Manilal VB Involvement of protozoa in anaerobic wastewater treatment process. Water Res. 41,20, Panicker, S.J., Philipose, M.C., Haridas, A Buoyant Filter Bio-Reactor (BFBR)-a novel anaerobi