The effect of shock loading on the performance of a thermophilic anaerobic contact reactor at constant organic loading rate
© Senturk et al.; licensee BioMed Central Ltd. 2014
Received: 18 March 2013
Accepted: 5 May 2014
Published: 12 May 2014
The influences of organic loading disturbances on the process performance of a thermophilic anaerobic contact reactor treating potato-processing wastewater were investigated. For this purpose, while the reactor was operated at steady state conditions with organic loading rate of 5.5 kg COD/m3 · day, an instant acetate concentration increase (1 g/L) was introduced to the reactor. During the shock loading test of acetate, it was observed that the overall process performance was adversely affected by all the shock loading, however, the system reached steady state conditions less than 24 hours of operation indicating that thermophilic anaerobic contact reactor is resistant to shock loading and be capable of returning its normal conditions within a short time period.
The wastewater from potato processing industry can be considered as a complex wastewater because of rather high concentrations of suspended solids, high content of insoluble COD fraction and significant quantities of potential foaming substances, such as proteins and fats [1, 2]. Therefore these wastewaters could only be discharged into municipal sewer system or receiving media after the reduction of the pollutants to acceptable levels. These wastewaters are usually treated with various combinations of aerobic and anaerobic biological processes due to high concentrations of readily biodegradable compounds [1, 3–6].
It is known that both the substrate retention time and the degree of contact between influent substrate and living microorganism population affect the performance of anaerobic reactors . Both of these parameters are a function of the mixing conditions ensured in the reactor. Mixing provides a suitable medium for the microorganisms to remain in suspension, as well as, for the biogas produced to leave the system . Additionally, mixing ensures heat transfer, and a homogeneous substrate distribution by preventing stratification and formation of surface crust .
Hydraulic retention time (HRT) is another important design parameter for digesters. For a given volume of wastewater, a shorter HRT is an indication of a smaller digester and, therefore, a more cost-effective solution. In order to reduce HRT, temperature or solid retention time (SRT) increase were applied previously . High-rate processes come forward to overcome this drawback of anaerobic treatment [10, 11].
The anaerobic contact reactor, a typical example of high-rate anaerobic processes, can be classified as the counterpart of the aerobic activated sludge process. Both reactors are characterised with a constant, mechanical mixing of substrate with recycled biomass. Anaerobic contact reactors have been used extensively in the food processing industry to treat typical high strength effluents with relatively high suspended solids [6, 12]. These reactors can be operated under different temperature ranges. Temperature can affect biochemical reactions in a number of ways, i.e. reaction rates increase with increasing temperature by the Arrhenius equation . Increased reaction rates would reduce retention times and therefore capital and operational costs would decrease. Moreover, increased organic solids destruction would decrease the waste sludge while yielding more biogas [6, 13].
The aim of this study is therefore to examine the effect of high acetate concentration on the performance and stability of the thermophilic anaerobic contact reactor (TACR). For this purpose, the most important operational parameters such as pH, alkalinity, total volatile acid concentration and biogas composition were monitored.
Materials and methods
Wastewater source and characterisation
The characteristics of the wastewater used (after peeling and cutting processes)
Total Kjeldahl nitrogen
Total volatile solid matter
Thermophilic anaerobic contact reactor configuration and operation conditions
In order to study the effects of different operational parameters, the TACR was continuously operated for over a year (Şentürk et al., 2010), before the organic shock loading study was carried out. In order to examine the effect of high acetate concentration on the performance and stability of the TACR, acetate concentration in the reactor was increased to 1 g/L instantly, when the reactor was operated at organic loading rate of 5.5 kg COD/m3 · day (HRT = 1 day). During the feeding of shock acetate loading to the system, no other changes were made in the raw wastewater characteristics or flow rate. The response of the anaerobic culture in the reactor to this high acetate concentration was then observed at constant organic loading rate.
All the chemicals used were of analytical reagent grade and water used during the experiments was laboratory distilled water. The analytical methods, which were used in order to monitor the performance of the system, were performed using the methods given in the Standard Methods . The COD and BOD5 analyses were carried out according to the STM 5220 C and STM 5210 B methods, respectively . The TKN analyses were performed using the STM 4500-Norg B Macro-Kjeldahl . The sulphate analyses were carried out using the STM 4500-SO42- method . The alkalinity and total volatile fatty acid concentrations were determined according to STM 2320 B and STM 5560 C methods, respectively . Acetic acid concentrations were conducted by a Gas Chromatography (Agilent) equipped with FID detector and a Zebran ZB-Wax capillary column, 30 m × 250 μm × 0,50 μm. Helium was used as the carrier gas. The oven temperature was initially set at 100°C for 1 min increasing 20°C/min to 120°C and then increasing 6.13°C/min to 205°C. The total duration was 15.87 minutes. The detector temperature was 240°C. The samples taken from the reactor were centrifuged for 15 minutes at 10000 rpm at room temperature and the supernatant of the sample was analysed accordingly. Additionally, the total solid matter and total volatile solid matter concentrations were also determined (STM 2540 B and STM 2540 C methods) .
The biogas produced was measured cumulatively using a gas-meter (Ritter) and the components (CH4, CO2, H2) were analysed by a Gas Chromatography (Agilent) using HP Plot Q + Molecular Sieve column, 60 m × 530 μm × 400 μm. Argon was used as the carrier gas with a gas flow of 4 mL/min. The oven temperature was initially set at 50°C for 5 min increasing 5°C/min to 80°C and kept at 80°C for 3 minutes, then increasing 10°C/min to 100°C. The total duration was 16 minutes. The temperature of TCD (Thermal Conductivity Detector) was 200°C.
Results and discussions
This study investigated the adverse effects of high acetate loading on a high-rate anaerobic contact reactor operated under thermophilic conditions. The organic shock loading was introduced to the system by dramatic increase of acetate concentration. It should be noted that before the organic shock loading study, the reactor was continuously run under steady-state conditions for over a year. The findings are discussed in the following sections.
pH and alkalinity
During the operation of a digester, pH is one of the most important factors and it is well known that anaerobic microbial activity is the highest in the pH range of 6.8 – 8.5 [15–17]. Additionally, the alkalinity of the anaerobic reactor should be maintained as close to the operating range as possible and there might be a need for addition of alkaline solutions in order to adjust pH especially during the acetogenesis phase. The pH fluctuations can affect both the bacterial growth and their activity in organic matter degradation adversely . Therefore, to keep pH in a specified region is of importance.
Volatile fatty acids
Biogas composition varies depending on feedstock, digester type and chemical addition . Additionally, the amount of biogas produced during anaerobic digestion depends on the feed organic matter content, the total volatile solid matter and the C/N ratio. The most known composition of biogas has about 60% methane, 35% carbon dioxide and about 5% the other gases [24, 25].
However, fluctuations in biogas composition started again after 7 hours of operation and these fluctuations continued for about 20 hours. Only after 14 hours, methane percentage in biogas started to increase. However, the negative impact of high acetate concentration on methane bacteria did not last long and steady state conditions were reached after the first 20 hours of operation following shock loading.
One of the most important features of the anaerobic contact reactors is that they are completely mixed reactors having a settlement tank. Anaerobic reactors, with continuous mixing facilities, are considered to be high-rate reactors from the point of mass transfer between substrate and microorganisms. Owing to this characteristic, the anaerobic contact reactor used in this study was found to be less affected by organic shock loading. When overall data were evaluated, thermophilic anaerobic contact reactor was found to be resistant to shock loading and can become stable only about in 20 hours.
This study was supported by the Gebze Institute of Technology Research Fund (Grant No. 2008-A-21).
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