Influence of organic loading rates on the production of methane from anaerobic digestion of sewage concentrate

This study investigated the efficiency of biogas production from sewage concentrate through anaerobic digestion. A continuous stirred tank reactor with a 900-mL working volume was used. The experiment was designed to investigate the influence of organic loading rate on the efficiency of biogas production and to determine the most suitable organic loading rate condition for methane production from sewage concentrate by using continuous stirred tank reactor. The reactor was operated at different organic loading rates of 1.8, 0.8, and 0.6 gCOD/(L.d). The methane composition of the biogas produced from the treatment organic loading rate (OLR). The beginning of the experiment recorded low methane production because of the high organic loading rate. However, the later part of the experiment recorded high and stable biogas production because of the relatively low OLR. Results suggested that a 0.6 gCOD/(L.d) OLR was the most efficient setup parameter for ideal methane production from sewage concentrate by using continuous stirred tank reactor.

Introduction

Managing waste is closely linked to the protection of the natural environment, where waste cannot be prevented. Several technologies have been introduced to treat and produce energy from waste. Each of these may have a role to play, given the variety of waste arising and the local situation in environmental issues arising from global environmental change, account for the emergence of anaerobic digestion (AD).

Land contaminated with sewage concentrate produces several environmental issues such as volatile organic compound emission, plants toxicity, and public health hazards.² Given the increasing costs related to waste disposal and energy supply, conversion of sewage concentrate and other biogas potential of waste to energy is becoming economically feasible.^3-5 AD has become a proven technology for managing both organic and solid waste.^6-7 Apart from generating biogas for energy use, AD also eliminates pathogens and produces end products that can be used as natural fertilizers for soil enhancement.^8-9

AD provides several environmental benefits such as energy production, soil nutrient improvement, and waste reduction. The biological treatment of the easily degradable organic components in sewage concentrate with high moisture is enhanced through AD. The digestion process is summarized into three steps, namely, solubilization, acidogenesis, and methanogenesis,^10-11 and involves continuous bacterial reaction.¹² During the first stage, solid substances such as food waste are solubilized for adequate degradation by continuous microbial digestion.^13-14 During the second stage, acidogenic bacteria ferment mainly volatile fatty acids (VFAs). During the last stage, methanogenic bacterial action on the degradable substrates produces methane and carbon dioxide.¹⁵ During each stage, the gas production and decomposition rates of organic waste are influenced by environmental factors.

AD technology is the most environmentally friendly and preferable treatment option for wastewater, sewage sludge, and domestic waste.^16-17 Sewage sludge only as substrates for AD technology requires a relatively straightforward process, and the treated sludge biosolids can be directly used for agricultural purposes.

Several factors affect AD. Some are related to the characteristics of substrates, operational conditions, and reactor design. The organic loading rate (OLR) is a very relevant parameter because it indicates the quantity of volatile solids to be put into the digesters daily. Volatile solids are that portion of organic components that can be converted to methane gas through digestion, while the remaining solid component is the digestate which can also be used in agriculture as fertilizer. The loading rate of digestion process depends on the type of wastes fed into the reactor because the type of waste influences the level of biological activity that occurs in a digester.

In this study, direct concentrated municipal sewage from enhanced membrane coagulation reactor (E-MCR) was used in a lab-scale mode using continuous stirred tank reactor (CSTR) to convert the substrates to biogas with different OLR conditions. From the numerous advantages of AD process for resource recovery, it is important to develop ideas for the maximum recovery of energy, reusable effluent, and nutrients as fertilizers from the treatment process in full scale. The objective of this study is to evaluate the bioenergy production of small community concentrates collected from E-MCR in CSTR with different loading rate.

Materials and methods

The digester experiment was conducted in a CSTR with a total volume of 1000 mL and a working volume of 900 mL. The reactor was supported with a rotor plate at the bottom which supports the continuous mixing of the digestion process. A gas sampling valve was located at the top and one outlet at the bottom for digested effluent removal. The reactor temperature was maintained at 38°C. Figure 1(b) depicts the schematic diagram of the CSTR, and Figure 1(a) depicts the experimental setup.

Figure 1.

(a) Experimental setup. (b) Schematic representation of the CSTR (1 is the digester, 2 is the sampling valve, 3 is effluent outlet, 4 is feed inlet, 5 is the mixer, 6 is the gas collector, while 7 is the gas outlet.¹

CSTR is referred to a kind of reactor that enhances the completely mixture of the substrate in the reactor and has the function of feeding and discharging of the substrate depending on the operational conditions. It is noted that microbes and wastewater have the same retention time; hence, hydraulic retention time (HRT) equals solid retention time (SRT). During CSTRs experiments, it is assumed that there is always perfect mixing in the system. However, in a good CSTR experiment, the output substrates are similar to that of the materials inside the reactor, which is said to be the function of residence time and rate of reaction in the system. In practice, mechanical or hydraulic agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations. CSTR has been used successfully for the digestion of sludge and slurry.¹⁸ The technique is also applicable for sewage sludge digestion at mesophilic temperature ranged between 30°C and 35°C. Mesophilic temperatures is applicable when the substrate is concentrated high enough to yield biogas.

Sewage concentrate sources and characteristics

The sewage concentrate used in this study was collected from the Xiao Jiahe municipal wastewater treatment plant. The sewage concentrate consisted only of domestic wastewater and no mixture of industrial wastewater. It had an average pH value of 7.64, average temperature value of 18.7°C, and chemical oxygen demand (COD) value of 7788 mg/L. The inoculum sludge used for the startup of the experiment was also collected from the Xiao Jiahe wastewater treatment plant. Table 1 shows the characteristics of the sewage concentrate used in this experiment and table 2 shows experimental set-up

Table 1.

Sewage characteristics.

	Concentration
	Minimum	Maximum	Average
pH	6.77	7.62	7.64
COD (mg/L)	7400	8700	7788
Temperature (°C)	17.2	20.9	18.7

COD: chemical oxygen demand.

AD experiment

The experiment was conducted in a continuous mode with daily feeding and discharging. AD was performed in a mesophilic temperature condition of 38°C with three different OLRs of 1.8, 0.9, and 0.6 gCOD/(L.d) and a constant retention time of 30 days. The retention time of 30 days was maintained by feeding 30 mL of concentrate and removing 30 mL of digested effluent every day. Table 2 shows the AD process under experimental conditions.

Table 2.

Experimental set-up

Period	Temperature (°C)	OLR (gCOD/(L.d))	Reactor volume (mL)	Discharge rate (mL/d)
Loading rate 1	38	1.8	900	30
Loading rate 2	38	0.9	900	30
Loading rate 3	38	0.6	900	30

OLR: organic loading rate; COD: chemical oxygen demand.

Analytical methods

The biogas production volume was measured frequently using a gas meter, and the gas composition was estimated regularly using an Agilent 7890A gas chromatography system with a thermal conductivity detector and a 2.0 mm stainless column. Ammonia and soluble COD were also determined using a membrane filter. The samples were filtered and subsequently tested according to the standard methods. Potassium dichromate was used as the oxidant for the COD test. Digestion was performed on the sample with a set amount of oxidant. The other tests were performed according to the standard methods.

The optimum feed range in liters per day was determined by measuring the gas production and composition. The daily produced gas was stored in gas bags and measured using syringe. Temperature is one of the most important factors in the digestion process because it controls the metabolic rate of the biosystem. Hence, the mesophilic temperature range was maintained at 38°C to ensure a balanced condition between the biogas yield and process stability. The optimum pH value of methanogenic bacteria was 6.8–7.5. Acidification was avoided using a hydrogen carbonate–carbon dioxide–carbon buffer system, which was created by the equilibrium between dissolved carbon dioxide and hydrocarbon acid. We evaluated the pH conditions, and the initial pH was set at 7.2 before the beginning of the experiment. To run a controlled experiment, all parameters were set at the known optimum conditions to set a base line.

Results and discussion

Biogas and methane production

One of the key objectives of this study was to investigate the AD performance when operated at different OLRs. Therefore, assessing the process performance according to biogas production and composition to the selected OLRs was relevant. Figure 2 depicts the biogas production during AD at three different OLRs. The daily biogas producing rate recorded from the three processes were approximately 34, 172, and 194 mL/(L.d). Low biogas production was recorded in the first scenario due to the high loading rate; some organic matters require a long degradation time, and the accumulation of undigested substrates results in a high VFA and subsequently low biogas production. Methane gas composition was within 21%–25%, 49%–58%, and 60%–68% accordingly from the three scenarios. Figure 3 depicts the biogas composition throughout the experiment period. The low methane composition in the first scenario was caused by the high content of undigested materials; as a result, VFA and acidifying microorganisms accumulated. However, the low OLR in the second and third scenarios influenced the increase in methane gas production.

Figure 2.

Average biogas producing rate from the three different conditions (1 = during OLR of 1.8 gCOD/L.d, 2 = during 0.8 gCOD/L.d, and 3 = during OLR of 0.6 gCOD/L.d).

Figure 3.

Methane compositions throughout the period of operation.

Biogas production rate per day

Biogas production rate accounts for the quantity of biogas produced from each reactor daily. In this experiment, the rate of production from the three treatment processes remained low from the first 25 days of the experiment, which is referred to as the startup period or the initial period of the operation where the microbial activities and the condition were not stable. According to the previous literature, it takes equivalent of 20 days for bacterial to settle and adapt to the environment and temperature conditions before the digestion process can properly function and become stable.

Production rate increased rapidly in scenarios 2 and 3 but very little in scenario 1. This is because it takes some organic substances longer time to degrade; shorter HRT results in lower biogas production while longer HRT enables the organic substances that are not easily degradable to degrade in a subsequent time and convert to biogas.¹⁹In this study, highest production rate was recorded in scenarios 2 and 3 with the highest production period ranging from day 60 to 90 of the experiment time. In scenario 2, highest biogas production rate was observed with the highest production rate of 452 mL/(L.d), the highest biogas production period in scenario 3 ranged between day 48 and 65 with the highest biogas production rate of 380 mL/(L.d).

Biogas yield in the CSTR

Hansen et al.²⁰ stated that higher HRT assists to improve the performance and biogas yield in CSTR system. It is important to note that HRT is closely related to the volumetric loading rate expressed as

HRT = V_{fermenter} [L] / V_{Feed} [L / d]

According to the literature, every gram of COD yields 0.35 L of methane at suitable temperature, where the produced biogas constitutes about 65% to 75% of methane. Since most of the volatile suspended solid (VSS) contained in sewage concentrates remained after the AD process, methane yield per gram of COD was highly influenced by the CSTR HRT. The biogas yield per gram COD of scenario 1 was 10 mL/gCOD, while the biogas yield per gram COD in scenario 2 was 220 mL/gCOD, scenario 3 recorded highest biogas yields with the value of 295 mL/gCOD.

Methane composition

The methane composition in the reactor during the experiment is shown in Figure 3. The percentage of methane in a well-operated AD treating black water and sewage concentrate ranges from 58% to 65%. The methane content of biogas should be stable over time unless the digester encounters a problem. In this study, the methane composition was influenced by OLR. The high OLR resulted in the low methane production from the startup period; the methane composition ranged from 9% to 21%. The CO₂ composition during the startup period was higher than the later period when the OLR was reduced. The methane composition increased during the fourth week to 39%–53% when the OLR was reduced to 0.9 gCOD/(L.d) and fluctuated between 58% and 69% when the OLR was further reduced to 0.6 gCOD/(L.d), whereas the CO₂ composition reduced to 19%–20%. Therefore, the low OLR influenced the methane production in a well-operated AD by using CSTR.

Process efficiency

Degradation

The COD degradability in the experiment was analyzed based on the CODs of the feed substrate and digested effluent. In principle, microbes in a digestion system break down organic matter to produce methane. Therefore, the feed substrate COD is always higher than effluent COD because some of the components of the feed substrates undergo anaerobic biodegradation to produce gas leaving behind the components that are readily biodegradable. Results showed that during the startup period when the OLR was high, the digested effluent COD remained high compared with when the OLR was reduced. The digested effluent COD was affected by the high OLR because microbes have difficulty breaking down organic matter at a high OLR. This result was similar with the results in the literature that the breakdown of organic substrates during AD depends on the OLR. When the OLR is low, the biodegradability and biogas yield in the system are high. Figure 4 depicts the COD degradability during the treatment process.

Figure 4.

COD variation of feed substrate and discharged effluent during the three different OLR. COD: chemical oxygen demand.

pH variation in the reactor

Figure 5 depicts the pH variation during the treatment process. The AD of organic substrates requires a group of microorganisms to work together. Methanogenesis is the most sensitive to low pH if the pH variation reduces beyond the normal range over a period of time. Methanogenic bacteria responsible for biogas production will be highly affected; as a result, methane production is reduced.²¹ Hence, the reactors need daily pH monitoring. By daily testing, the pH can be monitored and adjusted if it exceeds below or above the normal range for suitable performance. One of the causes of pH fluctuation is the instability in organic loading and a very short HRT. In this experiment, the initial pH for the reactor ranged from 6.8 to 7.2, inoculums, sludge, and glucose were used to start up the reactor. Subsequently, all the gas produced during this period was flushed out to neutralize the reactors before the experiment. The pH in the reactors at this point was within 6.8. As shown in Figure 5, the pH in the CSTR maintained within 6.7–6.8 for four days. Gas production was slow during this period; the production increased as the pH continued to reduce to a point ranging from 6.2 to 6.6. However, gas production in the reactor reduced when the pH condition further declined to 6.9–7.09. A low pH can result in accumulation of VFA, which somewhat inhibits digestion, whereas a high pH increases free ammonia, which is toxic for the methanogenic populations. Hence, the sudden decrease in pH in the reactor from the beginning of the experiment was caused by the excess loading of the substrate in the system, as the microorganisms cannot feed or act with the loading set for this reactor. System failure was not observed, but gas production remained low throughout the period when OLR was 1.8 gCOD/(L.d). High biogas production was observed when the pH balanced between 7.0 and 7.4 from the 21st day of operation, and the system remained stable throughout the remaining operation period. This stability was caused by the OLR reduction in the later period of the experiment.

Figure 5.

pH variations from the treatment process.

Organic loading rateǁ

OLR represents the amount of volatile solids fed into the CSTR digester per day during the continuous feeding process.²² Biogas production from the digestion process was highly affected by OLR. Feeding a large volume of new substrates daily to the reactor resulted in changes in the digester’s environment and temporarily reduced bacteria activity during the beginning period of the experiment with high OLR. This low rate of gas production occurred in the process due to the very high OLR that led to a higher hydrolysis bacterial activity than the methanogenesis bacterial activity. The increased hydrolysis bacterial activity eventually increased the VFA production.²³ The pH of the reactor decreased; as a result, the hydrolysis process was slightly inhibited such that the restricted methanogenesis bacteria were not able to convert the much accumulated VFAs to methane, and the volume of gas production was relatively low. Figure 6 shows the OLR for the treatment process. The second and third scenarios were operated with a lower OLR than that of the first scenario. Biogas production was higher in the second and third scenarios than in the first scenario with high OLR because the microbial activity in the system was able to act and adapt to the loading rate and produced biogas in the process.

Figure 6.

Organic loading rate of the treatment process. COD: chemical oxygen demand; OLR: organic loading rate.

Ammonia variation in the CSTR

Ammonia is usually formed during AD process as a reduced or reduction product of microbial influenced biochemical degradation of non-protein or protein nitrogenous substance. However, ammonia concentration in AD depends on SRT of the system and also relates directly to solids destruction during the digestion process.

From the experiment, ammonia concentrations were found to be directly influenced by SRT and the breakdown of solids. Ammonia composition in AD process increased as the SRT increased while the nitrogen concentration decreased as the SRT increased. In principle, total ammonia in the system is produced during the digestion of substrates. The presence of ammonia can inhibit the digestion process and decrease its total performance if the composition is too high in the system. The concentration of T-NH₃ and total nitroge (TN)over 1500 mg/L has been reported to be inhibitory for digestion process.²⁴ However, in this experiment, the composition of total nitrogen in the CSTR system did not exceed or reach the inhibition point. The highest recorded ammonia concentration from scenario 1 was 680 mg/L, and the highest concentration of nitrogen was 1100 mg/L. The value of the concentration of ammonia and nitrogen did not have any effect on system performance since the value did not exceed the estimated value for inhibition.

The percentage composition of biogas was higher during the stable period of the experiment. In principle, the content of nitrogen and ammonia depends on how concentrated the feed substrate is and depends on SRT of the CSTR. From the beginning when the feed substrates COD was higher, the ammonia content of the discharged concentrates from the CSTR was 18,720 mg/L. However, the increase in ammonia did not result in inhibition of the system.

Ammonia concentration in AD with the range from 50 to 200 mg/L seems beneficial to the process, while concentration from 200 to 1000 mg/L does not have effect on the process. But if the concentration increases to 1000 mg/L and above 1500 mg/L, there is a possibility that inhibition will occur because this value is toxic for the microbial activity in AD.

Conclusion

AD is a promising method for reducing the amount of degradable sewage concentrate discharged into the environment daily by converting waste to renewable energy. Given the characteristics of sewage concentrate, AD provides a practical and essential method to convert biodegradable wastes to methane gas. The process showed stable methane production during the third scenario with 0.6 gCOD/(L.d) OLR; the methane proportion ranged from 60% to 68%. Results suggested that 0.6 gCOD/(L.d) OLR was the promising operation condition for ideal methane production from CSTR. AD, if successfully implemented as a method of sewage concentrate treatment, will help in reducing waste accumulation in the environment and serve as a renewable energy resource.