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Published: Tuesday, 10 June 2014 14:51

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Bioreactors: Anaerobic digesters

Gregory DiCenzo

Introduction

Bioreactors are reaction vessels inside which a chemical process occurs and the key reactions in that process are catalyzed by organisms. This paper will be concerned only with systems that have a significant anaerobic requirement. As operators are expected to reduce CO₂ releases and even recover energy from wastewater and other carbon sources, more effort will be given managing reactors for methane production (Kraemer et al., 2012).

Reactors come in a wide range of sizes: from 0.5 L to several cubic meters. They are commonly made of stainless steel, which allows cleaning in place. Features common to all or most anaerobic reactors are 1) gastight, 2) temperature controlled, 3) ports for influent/effluent, 4) gas or pressure release, 5) pH sensing and control, 6) agitation, 7) loading access and 8) sampling port.

Reactors are operated in either a batch or continuous mode. Batch operation implies that upon loading and sealing the reactor, neither the starting material or desired product is removed until the process and its reactions have reached a predetermined set point. Continuous operation allows for continuous flow of reactants into the vessel with simultaneous output of product.

Batch reactors should be sized larger to hold an economically useful amount of reactant. Continuously fed reactors can be smaller. The closed batch reactor maintains containment of all materials including the headspace atmosphere and is not dependent on influent and effluent pumps. The continuous reactor allows for monitoring and adjustment of reactants, products and inhibiting components.

Management and Operation

Management objectives may vary. In most cases, anaerobic reactors – also known as “digesters” – are operated chiefly to eliminate the carbon burden in a water phase. Thus, the main objective is to reduce the carbon load measured as “chemical oxygen demand” or “COD”.

Some waste water processes depend on aeration, that is, the intentional maintenance of dissolved oxygen (DO) in the fluid phase. In these systems, the aeration is usually accompanied by agitation and turbulence such that the organisms are best described as a “floc” which must be continuously regenerated. In the anaerobic scenario, the microorganisms or “sludge” will reside as a layer, sometimes called a “blanket” which is settled to the bottom of the reactor. Influent, which is high in COD, is pumped into the sludge for maximum contact.

The microorganisms consist of a community of diverse bacteria and Archaea and can physically associate as granules, which aids in the sedimentation, retention and overall efficiency. The granules make up the blanket and can slow solids transport while smaller dissolved sugars and volatile fatty acids (VFAs) are converted to gas and exit both the blanket and liquid phase more rapidly.

The first stage in anaerobic digestion is a hydrolysis process during which larger, complex molecules are reduced, forming smaller and more soluble products. Though this is an integral part of a wholly contained process, the progress of the first stage can be accelerated by implementing a pretreatment: which is both a physical and chemical treatment that increases surface area of the feed stock but can also decrease size while increasing solubility. The requirements for a pretreatment are minimal: an economic process carried out in a simpler containment system. Not all feed requires or can benefit from a pretreatment: only those which are composed fundamentally of larger and less soluble molecules.

Most anaerobic reactors have an upward flow of influent through a bed of granules; the residence time in the granules should be sufficient to allow an economic and functionalconversion of the feedstock to biogas (although some systems might separate stages of the process into sequential reactors; i.e. emphasizing methanogenesis in the last stage reactor).

A common example is the upflow anaerobic sludge blanket (UASB) reactor which is often used for wastewater. In this scenario, the blanket is composed of granulated biofilm which contains all the bacteria and archaea necessary for hydrolyzing the feedstock and producing methane. The upward flow allows for control of the granule-feed contact and process rate, leaving a much smaller COD burden in the effluent. There can be a net gain of energy in the form of biogas depending on downstream treatment requirements.

Modifications to the UASB can includea membrane to keep solids and biomass out of the effluent or a filter material which anchors the organisms within the tank but still keep them in the process stream.

Recent case studies in anaerobic digesters

Gallagher and Sharvelle (Gallagher and Sharvelle, 2011) tested the efficacy of a pilot scale (114 L) anaerobic digester in a UASB configuration to treat “black water” (BW, toilet water only). In this small pilot, organic loading rate was only 0.21-0.39 kg COD m³/day with a retention time of 2.6-4.0 days. COD was reduced 72% while TSS and VSS were reduced 95%. Of interest in this BW reactor: E. coli and fecal coliforms showed greater than 90% log reduction.

Hassan and Nelson (Hassan and Nelson, 2012), in their review of dairy wastewater anaerobic digesters, recommend UASB configurations with a pretreatment to increase bioavailability and reduce obstacles to smooth operation posed by milk fat. Awareness of the high fat content and special concern for negative outcomes from spikes in vfa’s and other fatty acids require careful management of hydraulic retention times and pH/buffering management. Efficiency of biomass conversion is typical for this technology as they report 0.22 – 0.40 m3 biogas/kg COD removed.

In their review of fish processing wastewater treatment Chowdhury et al. (Chowdhury et al.) discuss the complex and potent nature of waste from fishery processing and cooking. The water contains solids, high COD and oil and other fats. The wastewater can also be relatively high in volatile amines and other nitrogenous compounds (especially NH3⁺ at higher pH conditions). Fish waste conversion in reactor systems such as UASBs is efficient with loading rates around 1 kg COD/m3(per day) and retention times of 11-35 days. The authors recommend a post anaerobic aerated stage to polish the effluent. They report conversion efficiencies (COD to biogas) ranging from 60-70% in various anaerobic digester formats.

Agricultural waste ranges from manure to highly cellulosic biomass to fruit and vegetable process waste. Some of their unique challenges can be pretreatment requirements for the straws/grasses, and buffering for the high volatile solids feed stock that can otherwise over-acidify the reactor (Ward, et al., 2008). The requirements for a successful operation of an anaerobic digesting system include an alkaline pretreatment of straws to hydrolyze cellulose components and also to reduce the particle size. Buffering may also be required to counteract rapid spikes in vfa’s because, at pH < 6.6, methanogenic efficiency is strongly reduced. Indeed, they recommend managing of the buffering capacity rather than pH to anticipate and avoid system shocks. Yields for conversion of COD to biogas 0.14 (yard waste) to .472 (mixed food waste)m3 biogas/kg VS; 0.3 m3 biogas/kg VS (corn) to 0.19 m3 gas/kg VS (straw)

They also recommend two-stage systems that can separate the hydrolytic and acidogenic processes (pH 5.5-6.5) from the methanogenic stage (optimum pH 7.0). Also, when treating these high solid feed stocks, the microbial biomass should be fixed or anchored to a support such as is found in an anaerobic filter (AF) or use a mesh or membrane that retains granules.

Yu, et al. also promote 2 stages to separate the lower pH processes from the higher pH methanogenesis process. They treated high solid municipal waste (a “shredded paste” with high organic content) in a single reactor that maintained a stirred upper zone (which contained the floating solids) and alower zone that specialized in organic degradation. The effluent was ported to a UASB “seed” reactor which processed vfa’s to methane; most effluent was released from the UASB while some biomass was returned to the first stage (Yu, et al. 2012). The best operation (best rate of biogas production) maintained a particular recycling rate of UASB fluid which carried some methanogens but only about 0.14kg COD/day compared to a feed rate of total solids of 5 kg COD/day. The recycled feed was over-represented by vfa’s which could reduce the pH if allowed to flow at too high of a rate. This experimental system was operated primarily for data collection in order to model all processes. The relatively low rate of methane production was .022 m³ CH₄/kg COD per day.

Christian et al (Christian et al) describe a large, higher temperature (33°C), high throughput anaerobic membrane bioreactor (AnMBR) used to process high strength wastewater from a food processing plant. It processes 10 to 15 kg COD/m3·d while all necessary microbial biomass is retained by the membrane. Removals are >99% of COD and BOD; effluent contains 210 and 20 mg/L of COD and BOD respectively. That effluent is treated in a separate aerated process, although the sludge from that process is returned to the AnMBR for digestion.

Anaerobic hybrid reactors (AHRs) combine a UASB reactor in the lower portion of the reactor with an AF (anaerobic filter) above. During operation, the retention time can be shortened, which increases the upflow while the AF retains the solids and biomass (Ramakrishnan and Surampalli 2013). The authors of this study compared AHR operation at two temperatures: 35°C and 55°C. The synthetic feedstock simulated coal wastewater: water from liquefaction and gasification processes. The COD was 2240 mg/L and phenolics concentration of 752 mg/L. Coal gasification wastewater is considered to be relatively recalcitrant. (Wang et al. 2011). 0.48 m3 gas/kg COD for higher temp (55)

Operating replicate reactors (at 33°C and 55°C) the authors compared chemical removal and methane synthesis for different retention times. At each retention time, the thermophilic reactor always outperformed the mesophilic reactor with respect to COD removal, phenolic decomposition, specific methane yield and overall operational stability.

Full scale anaerobic reactors

A large AnMBR reactor in Massachussetts USA (Christian, et al. 2011) has an inflow rate or 475 m³/day (35 g/L COD). In its current form, it consists of 4 membrane-containing reactors and produces 5700-8000 m3 biogas/day (~.16-.23 m³ biogas/kg COD. This reactor at Ken’s Foods (Massachusetts, U.S.A.) has been upgraded several times because its output was exceeding daily discharge volumes as permitted, causing the food processing plant to halt production. In addition, the effluent TSS is ~ 1 mg/L well below the 600 mg/L permit.

Schievano, et al. (2011), in a compressive review of 3 full-scale anaerobic digesters, concluded that the reactors function at near lab-scale efficiencies after calculating the “biomethane potential” (BMP) and comparing to specific gas yields.

Bio-methane and biogas productions, process performances and efficiencies achieved by the three observed full scale biogas plants during the observation period (April 2008– March 2009). (Schievano et a., 2011)

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Plant A: municipal solid waste, ~74 kg/day; Plant B: swine manure and food process waste, 66kg/day; Plant C: swine/bovine manure plus silage, whey and rice. BMY₁ = (BMPin * TSin- BMPout* TSout)/(BMPin* TSin). BMY₂= SMP/BMP where BMP_in = biomethane potential. *Mg = 1000 kg.

Conclusion

Interest in anaerobic digesters draw more interest as greater value is placed on recovering energy from the waste carbon load in a process that also reduces CO₂ and methane releases. While most reactors described in the literature are UASB in configuration, AnMBRs or hybrid reactors may be better suited to the slower anaerobic process (Stuckey 2012). Upflow through a filter can shorten retention time while preserving biomass. De-nitrification may become more important as anaerobic processes become more important. Options for nitrogen removal include a separate side stream process or harnessing anaerobic ammonium oxidation activity within the reactor.

Full scale reactors reduce operating costs, decrease effluent TSS and generate biogas. In addition, sludge and solids are mitigated in this process and CO₂ and odor release is minimized.

References cited

Chowdhury, P., T. Viraraghavan , A. Srinivasan. 2011. Biological treatment processes for fish processing wastewater – A review. Bioresource Technology 101:439–449.

Christian, S., S. Grant, P. McCarthy, D. Wilson and D. Mills. 2011. The First Two Years of Full-Scale Anaerobic Membrane Bioreactor (AnMBR) Operation Treating High-Strength Industrial Wastewater. Water Practice & Technology Vol 6 No 2 doi:10.2166.

Gallagher, N. and S. Sharvelle. 2011. Demonstration of Anaerobic Digestion of Black Water for Methane Capture and Use in an Office Building. Water Practice & Technology Vol 6 No 1 doi:10.2166.

Hassan, A.N. and B. K. Nelson. 2012. Anaerobic Fermentation of Dairy Food Wastewater. J. Dairy Sci. 95:6188–6203.

Kraemer J.T., Adrienne L. Menniti, Zeynep K. Erdal, Timothy A. Constantine, Bruce R. Johnson, Glen T. Daigger, and George V. Crawford. 2012. A practitioner’s perspective on the application and research needs of membrane bioreactors for municipal wastewater treatment. Bioresource Technology 122:2–10.

Ramakrishnan, A., R.Y. Surampalli. 2013. Performance and energy economics of mesophilic and thermophilic digestion in anaerobic hybrid reactor treating coal wastewater. Bioresource Technology 127: 9–17.

Schievano, A. G. D’Imporzano, V. Orzi, F. Adani. 2011. On-field study of anaerobic digestion full-scale plants (Part II): New approaches in monitoring and evaluating process efficiency. Bioresource Technology 102:8814–8819.

Stuckey, D. 2012. Recent developments in anaerobic membrane reactors. Bioresource Technology 122:137–148.

Wang, W., Wencheng Ma, Hongjun Han, Huiqiang Li, Min Yuan. 2011. Thermophilic anaerobic digestion of Lurgi coal gasification wastewater in a UASB reactor. Bioresource Technology 102:2441–2447.

Ward, A.J., Hobbs, P.J., Holliman, P.J. andD.L. Jones. 2008. Optimisation of the anaerobic digestion of agricultural resources. Bioresource Technology 99:7928–7940.

Yu, L.,Q. Zhao, J. Ma, C. Frear, S. Chen. 2012. Experimental and modeling study of a two-stage pilot scale high solid anaerobic digester system. Bioresource Technology 124:8–17.