2.1. Industrial Wastewater Types

Industrial wastewater varies in composition and cannot be simply characterized. Three major types of wastewater can be defined, processing, cleaning and sanitary. Different industrial sectors generate different compositions and quantities of these wastewaters at the effluent^[3]. It may be highly biodegradable or not at all and may or may not contain compounds recalcitrant to treatment. The main concern with industrial wastewater is the increasing amount (in quantity and variety) of synthetic compounds contained in and discharged to the environment. The main industrial wastewater types are presented in Table 1.

2.2. Industrial Water Suitable for Anaerobic Treatment and Methane Production

Anaerobic techniques are typically used for high-loaded wastewaters, expressed in chemical oxygen demand (COD), typically greater than 1500–2000 mg/L. The application of anaerobic wastewater treatment is largely confined to relatively heavily polluted wastewater with a COD between 3000 and 40,000 mg/L, e.g., in the sugar, starch, fruit and vegetable and alcoholic drinks sectors. There has recently been some success in using certain anaerobic systems even for less heavily polluted wastewater with a COD between 1500 and 3000 mg/L, e.g., in breweries, dairies and in the fruit juice, mineral water and the soft drinks sectors. Assessment of methane production potential from industrial wastewater streams is based on the concentration of biodegradable organic matter in the wastewater, Table 2 and Table 3 ^[5][6].

The volume of wastewater, and the propensity of the industrial sector to treat their wastewater in anaerobic systems are also important. Using these criteria, major industrial wastewater sources with high methane production potential can be identified as follows:

The number of wastes that are amenable to anaerobic digestion is quite large (Table 2,3). However, the feasibility of the anaerobic digestion of an industrial waste is determined by several factors. These factors are waste concentration; waste stream temperature; anaerobic process inhibitors presence in waste stream; expected biogas yield and treatment efficiency^[5]. The anaerobic process has many advantages over conventionally used and proven aerobic processes. The most significant positive of the technology is the fact that energy is generated in the form of biogas by means of anaerobic decomposition from unnecessary waste in the form of pollutants in the wastewater.

2.3. Composition of Selected Industrial Wastewater

The composition of industrial wastewater will be characterized by both the substance load and the proportion of biodegradable substances and the values of other, more specific parameters (Table 4).

3.1. Aerobic Versus Anaerobic Wastewater Treatment

 When aerobic and anaerobic treatment is compared, some advantages which bring appropriate application of anaerobic technology can be found (Table 5). Anaerobic treatment produces up to ten times less biomass, which is related to very low requirements for nutrients and energy in the form of biogas. Excess sludge does not need to be further stabilized, which is a necessary part of aerobic treatment^[7].

From the bioreactor and the overall cleaning process point of view, anaerobic wastewater purification is an alternative to aerobic technologies. The anaerobic process has mainly been used for sewage sludge stabilization. In anaerobic reactors, a high concentration of biomass is still maintained, which is related to the ability to treat wastewater with a very high material load. On the other hand, anaerobic microorganisms grow very slowly, which is why it is still necessary to maintain their high concentration and leave the substrate in the anaerobic reactor for a long time >15 days^[5]. However, one of the biggest advantages of this is that the high sludge retention time in the reactor promotes the growth of microorganisms with a long generation time, which are able to degrade pollutants that cannot be degraded in aerobic conditions or are toxic to aerobic microorganisms. Finally, we cannot omit the price of operation, which is significantly lower for anaerobic technologies^[7].

Anaerobic treatment also has its disadvantages. The total loss of organic matter is still not achieved in anaerobic processes and it is often necessary to treat the wastewater aerobically. Another major disadvantage is the high sensitivity of anaerobic organisms, especially methanogenic archaea, and their long generation time, which is associated with a long time to start the operation of reactors.

3.2. Anaerobic Digestion

Figure 2. Scheme of reactions during anaerobic decomposition of polymeric materials^[11], edited by authors.

The anaerobic process usually takes place in mesophilic (35–50 °C) or thermophilic (50–60 °C) cultures of microorganisms, and, in terms of biogas yield, thermophilic cultures show higher biogas production. The biogas produced consists mainly of carbon dioxide and methane. Typical methane content in biogas ranged from 50 to 85%vol. Other gases (H₂S, NH₃ and water vapor) are usually present in biogas in the hundreds or thousands of ppm. In addition to methanogenic communities, bioreactors can also contain bacteria that can compete with methanogens for the available substrate and thus negatively affect biogas production. These are facultatively anaerobic bacteria that can use molecular oxygen as an electron acceptor, denitrifying bacteria that use nitrates, sulfate-reducing bacteria (SRB) using sulfides and sulfates, or bacteria that reduce iron ions. 

3.3. Anaerobic Bioreactors

Anaerobic processes for the treatment of wastewaters and sludges are well over 100 years old^[12]. In the last 50 years, anaerobic reactor technology evolved from localized lab-scale trials to worldwide successful implementations at a variety of industries. Since the 1970s, when Lettinga et al.^[13] first described the Upflow Anaerobic Sludge Blanket (UASB) process, a lot of progress has been made in the anaerobic wastewater treatment. Today, different types of high-rate anaerobic reactors characterized by short hydraulic retention time, high pollutant removal efficiency and high applicable volumetric loading rates are commonly used across the industry worldwide.

Anaerobic bioreactors initial drawbacks (biomass slow growth rate, susceptibility to toxic compounds, huge reactors volumes, etc.) were overcome. The new generation of reactors with a short hydraulic retention time (HRT) (2 h to 48 h) and the ability to process high organic loading rates (4 to 40 kg COD/m³ reactor per day) was developed (Table 6). In these reactors, the problem of slow growth rate was turned by capturing the biomass in the form of biofilms on static or moving supports but also by selecting well settling flocculating biomass. This development resulted in much smaller reactors volume but also in a much more stable operation than before.

Most widespread technologies for anaerobic wastewater treatment reactors are upflow Anaerobic Sludge Blanket Process (UASB), Internal Circulation (IC) and Expanded Granular Sludge Bed (EGSB) (Figure 3A–C). The UASB reactor is a suspended-growth reactor that maintains a very high concentration of microbial biomass by promoting granulation. The anaerobic granules are 1–3 mm in diameter and dense enough to settle down in the reactor. The biomass concentration in the UASB reactor reaches 50 g/L or higher and thus maintains a very long sludge retention time >15 days irrespective of the short hydraulic retention time of 4–12 h. Upflow velocity typically ranges from 0.5 to 1.0 m/h, 4.5 to 6.5 m high^[14]. The EGSB reactor was developed from UASB reactors. It has a high recycle ratio, and the upflow of this reactor is typically maintained higher than 6 m/h, 12 to 16 m high^[14]. The IC reactor is a new concept being mostly used for the treatment of industrial effluents of high strength. The higher OLR of the IC reactor is mainly due to its internal circulation, which allows for an improved contact between the biomass and the influent^[14].

Figure 3. Scheme of an Upflow Anaerobic Sludge Blanket (UASB) reactor (A), an Internal Circulation (IC) reactor (B) and an Expanded Granular Sludge Blanket (EGSB) reactor (C).

Many reactors operate despite the same names on the same or similar principles. Due to the evolving technology, only specific modifications are made to the basic structures due to several design companies. The potential of anaerobic reactors in wastewater treatment is now great but will increase as the parameters are unified and the technology is universalized.

4. Methanogenic microorganisms in industrial wastewaters

The composition of microorganisms in anaerobic bioreactors will be affected by many factors. These are, reactor design, temperature, pH, C:N ratio, wastewater composition, organic loading rate, hydraulic retention time, agitation^[5]. A very important fact is that anaerobic bioreactors during start up are usually seeded with inoculum from other biotechnology. Wide range of anaerobic bacteria are reported inhabiting anaerobic fermenters. These belongs mainly to the Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes, Spirochaetes, Chloroflexi, Planctomycetes a Synergistes. Archaea represents approximately 5-6% of total microbial population in anaerobic fermenters.

4.1 Inhibitors and methanogenic activity

Anaerobic digestion is particularly susceptible to the strict control of the environmental conditions, as the process requires an interaction between fermentative and methanogenic organisms ^[5]. Anaerobic biodegradability and methanogenic toxicity are strongly dependent on wastewater characteristics. Toxic substances present in wastewater can interfere with the metabolism of readily biodegradable substrates^[15]. There is a lot of industrial wastewaters amenable to anaerobic biotechnology. Nevertheless, these effluents may have properties that can cause process inhibition. The main influencing properties are temperature, pH, alkalinity, volatile acids concentration, redox potential, salinity, macro and micro nutrient deficiency, presence of specific cations (Ca²⁺, Na⁺, K⁺, Mg²⁺, NH₄⁺), hydrogen, sulphide, heavy metals, bleaching and dyeing agents, antibiotics.

5. Methanogens presence in selected industrial wastewaters

Anaerobic wastewater treatment is highlighted in many sources as a very efficient and ecological alternative to aerobic technologies. Today, this technology is already used in many industries, especially in the food industry. Based on the literature review, it is possible to conclude at least that a relatively permanent part of the community of methanogenic archaea in anaerobic sludge, with a few exceptions, are genera Methanothrix and Methanosarcina, acetoclastic representatives of methanogens (Table 7). The diversity of methanogens is reflected in the different growth conditions, temperature, pH and osmolarity. Most of the methanogens grow optimally around neutral pH. Higher wastewater pH can be tolerated by halotolerant strains of methanogens. For example, in dairy wastewater Methanocalculus sp. was reported as part of methanogenic community. On the other hand, moderately acidic environments can be inhabited by methanogens as, for example, Methanoregula sp. Methanosarcina sp. and Methanococcus sp. Salt concentration may also be an important physiological parameter for methanogens. In high salinity brewery, paper and dairy wastewaters representants of Methanobacterium sp., Methanosarcina sp. and Methanothrix sp. can be found. We can prove the theories that Methanothrix sp. plays a key role in granulation and in the core of the granules function as nucleation centers that initiate granule development. 

Table 7. Methanogens prevailed in anaerobic bioreactors treating industrial wastewater