REVIEW

Anaerobic digestion technologies for the treatment of pig wastes

Tecnologías de digestión anaerobia para el tratamiento de residuales porcinos

Tania Pérez- Pérez,^I Ileana Pereda- Reyes,^II Deny Oliva-Merencio,^III Marcelo Zaiat,^IV

ABSTRACT

The growth of pork production in the world favors the excessive generation of manures, so that anaerobic digestion plays an important role as a technology for the treatment of these wastes. The literature shows that in the world, in the last years, the application of high load technologies for the treatment of wastes generated by the pig breeding has increased. In contrast, the anaerobic biodigesters used in Cuban pig breeding currently use high hydraulic retention times and low volumetric organic loads. This involves the use of reactors with high volumes, which lead to huge investments for the treatment of these wastes, which makes these technologies inefficient. It is necessary, from a scientific perspective, to examine the technologies that are used today in the world and to develop researches that allow the fulfillment of the established in the Cuban standards in force on this subject.

Key words: anaerobic digesters, pig breeding wastes, high load technology.

RESUMEN

Palabras clave: digestores anaerobios, residuos de porcinocultura, tecnologías de alta carga.

INTRODUCTION

During the last decades, the number of small pork producers has decreased, while the animal population per farm has increased (Deng et al. 2014). These conditions have led to a higher concentration of waste in small areas, which requires urgent development of easy-to-implement solutions.

Among the potential solutions for wastes treatment the anaerobic digestion (AD) deserves special attention due to its efficiency and economic feasibility (Sakar et al. 2009), as well as its potential for the bioenergy (CH4) production, reduction of emissions of greenhouse gases and deactivation of pathogens (Massé et al. 2011, Abbasi et al. 2012, Franke-Whittle et al. 2014).

In recent years, significant progress has been obtained in the world through the evolution of high-rate anaerobic reactors, which deal with high volumes of wastewater. Due to their different advantages, the most used include the anaerobic filter (AF), anaerobic sequential broken reactor (ASBR) and the upflow anaerobic sludge blanket reactor (UASB). However, in Cuba, today, the most used technologies for the treatment of pig wastes comprise low-load reactors, which are inefficient due to their high hydraulic retention times (HRT), which lead to high volumes of reactors and high costs.

The objective of this study is to make a review about the anaerobic technologies applied worldwide in the treatment of pig wastes. It is also intended, to examine the particularities of this theme in Cuba.

The world pig production has increased more than 3.5 times over the past 40 years. The tendency is to increase the concentration of animals per farm, even reaching values of thousands of heads. As the demand of pork meat increases, huge amounts of waste are generated around the world, which is associated with increased excreta and wastewater (Lee and Shoda 2008).

In previous decades, pig wastes were used as fertilizers and soil conditioners in which they were generated. However, the intensive cattle rearing has caused serious damage to the environment, including health risks, atmospheric emissions, surface and groundwater contamination, odor dispersion and soil damage (Massé et al. 2004, Lim and Fox 2011). Due to the characteristics these wastes have.

Among the atmospheric emissions, the greenhouse gases (methane and nitrous oxide) are highlight, which are the result of spontaneous self-purification of wastes. In turn, the action of manures and slurries in waters and soils is mainly concentrated, in the dispersion of ammonia and nitrates, due to their potential effect on the acidification of the environment and water eutrophication (Benyoucef et al. 2013).

Extensive are the studies carried out in the characterization and treatment of pig wastes, in order to reduce their negative effects on the environment. Andreadakis (1992) showed that, approximately, 60.0 % of the total organic matter in the pig wastewater is biodegradable. Similarly, Deng et al. (2008) reported that pig manure is composed of readily biodegradable material, and is a good base substrate for biological processes, such as anaerobic digestion, since it contains buffer capacity and a wide variety of nutrients (González-Fernández and García-Encina 2009, Rodríguez et al. 2011). These results show that the compounds present in pig manure contribute to the concentration of biochemical oxygen demand (BOD), the main indicator of the organic load responsible for the pollutant power of these wastes.

Other indicators that cause contamination in the wastes from pig facilities, due to their high concentrations, are suspended solids, chemical oxygen demand (COD), nitrogen, phosphorus and potassium compounds (Girard et al. 2004, Klomjek 2016) and gaseous substances such as ammonia, methyl mercaptan and hydrogen sulfide (Rumsey et al. 2014).

As is showed, pig wastes have great pollutant power and require an effective treatment before being poured into receivers. Due to the high concentration of biodegradable compounds present in these wastes, the anaerobic treatment can be an effective option to contribute to the environment care.

ANAEROBIC DIGESTION FOR THE BIOGAS PRODUCTION

The anaerobic digestion (AD) is the biological conversion, in the absence of oxygen, that make the facultative and anaerobic bacteria and methanogenic archaeas, from the synthesis of organic matter present in solid wastes (domestic and urban), from animals and plants, and its transformation into methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), hydrogen sulfide (H₂S) and nitrogen (N₂) (Batstone and Jensen 2011). This gas mixture is called biogas.

The initial step of the Ad process is the hydrolysis of organic matter. In this one, bacteria and fungi convert macromolecules (proteins, carbohydrates and fats) into amino acids, sugars and volatile fatty acids (Nielsen et al. 2007, Leis et al. 2014). In the second stage of the process, called acidogenesis, the acidogenic bacteria turn sugars, amino acids and fatty acids into organic acids, alcohols and ketones, acetate, CO₂ and hydrogen. The acetogenic bacteria transform fatty acids and alcohols into acetate, H₂ and CO₂ in the third stage of the process (acetogenesis). These products are used by methanogenic archaeas to form biogas in the fourth stage of AD, called methanogenesis (Pereda et al. 2015). In most of the anaerobic treatments of biodegradable wastes, this last step is limiting and therefore, it defines the main parameters of monitoring and control in the process. Methane can be produced by acetotrophic and hydrogenotrophic methanogens. The changes in the environmental and operational conditions of the reactor (pH, temperature, hydraulic retention time, substrate composition) influence on the composition and dynamics of these communities (Kim et al. 2013, Solli et al. 2014, Yu et al. 2014).

It can be explained that the AD of pig wastes is important because of its environmental and energy effect. Due to the rising fossil fuel costs and the need to mitigate anthropogenic global warming, the biodegradable waste AD is a sustainable management strategy. The biogas production from several types of raw materials has proven to be a renewable source of energy that can be produced sustainably in many countries. Likewise, the generation of an effluent with fertilizing characteristics, gives this process higher relevance. However, the effective treatment of wastes should consider the use of appropriate and economically feasible technologies to comply the environmental standards established in each country.

ANAEROBIC DIGESTION TECHNOLOGIES USED IN THE WORLD FOR PIG WASTES TREATMENT

Anaerobic processes are widely used for pig wastes treatment in reactors of various designs and scales, with the purpose of stabilizing the organic matter with the consequent production of energy in methane form (Hwang et al. 2001, Angenent et al.2002, Ahring 2003). The most studied are AF (Oleszkiewicz 1983, Ng and Chin 1988), ASBR (Mace and Mata-Alvarez 2002, Massé et al. 2003,  2004, Kim et al. 2004, Ga & Ra 2009) and  UASB (Lo et al. 1994, Sanchez et al. 2005, Song et al. 2010). The latter are the most used on an industrial scale. Other types of high-load technologies, such as EGSB, have also been considered for the treatment of these wastes, but on a lower scale (López-Fernández et al. 2011, Lee and Han 2012). These technologies have as main objective to achieve high of elimination efficiencies, when treating higher volumetric organic loads (VOL) in the lower reactor volume, which guarantees lower HRT with high retention of active biomass (SRT).

The AF is characterized by being robust systems, with good retention of microorganisms in the medium, which can work at high VOL and low HRT. Oleszkiewicz (1983) studied the treatment of pig wastewater with an AF at 23 °C and obtained removal efficiency of COD of 73.0 % at a VOL of 4.0 kg COD / m³d. Ng and Chin (1988), when treating pig wastewater with an expanded bed anaerobic filter at 30 °C, reported total suspended solids (TSS) and volatile (VSS) of 93.0 % in a HRT of five days. However, the removal efficiency of TSS decreased dramatically to 74.0 % when the HRT decreased to four days. Regardless of its advantages, this configuration is limited to low values of ascending speed, to avoid the dragging of solids with the effluent. This does not allow, sometimes, a good biomass-substrate contact and the efficiency can be affected in some cases.

Likewise, the ASBR is a technology that highlight for its flexibility in operation. Its main advantages are that the reaction and sedimentation occur in the same unit, and as a consequence the biomass is in a dynamic state of abundance. These characteristics make possible that these systems treat solids and liquids waste, and the operating and maintenance costs be minimal (Islam et al. 2011). There are several experiences with this type of reactor. Massé et al. (2003) used an ASBR at laboratory scale to treat pig excreta and they observed that removal efficiency of COD decreased with decreasing temperature. The values obtained by these authors were 94.2 ± 1.1 % at 20 °C, 78.8 ± 3.0% at 15 °C and 60. 4 ± 6.4 % at 10 °C. but Massé et al. (2004) in a pilot-scale reactor obtained removal efficiency of COD of 79.5% at 20 °C. Likewise, Ng (1989), with this same configuration, but at a 28 °C temperature, obtained removal efficiency of VSS of 89.0 % at a VOL of 0.4 kg COD / m³d. However, when the latter increased, the efficiency was markedly deteriorated. Similarly, Ndegwa et al. (2008) studied an ASBR at different temperatures and obtained that the biogas production remained practically constant at 20 and 35 °C. However, the COD removal slightly decreased from 85.0 to 80.0 % with increasing temperature.

According to the above results, it can be inferred that temperature, such as the VOL and the HRT, has a marked influence on the removal efficiencies of solids and organic matter in the mentioned configurations. For this reason, Zaiat et al. (2001) recommended increasing the solids retention time (SRT) to improve the organic matter removal in ASBR reactors.

Many researchers have reported satisfactory performances in the treatment processes of pig wastes with high-rate reactors, based on sludge granulation where high biogas quality is obtained in the range of 70.0-80.0% of the methane content (Mahmoud et al. 2003, Kim et al. 2004, Najafpour et al. 2006). These reactors allow the accumulation of high biomass concentrations in granular sludge form, which results in a process of high efficiency and stability, with short HRT (McHugh et al. 2003a, 2003b, Najafpour et al.2006).

In particular, the upflow sludge blanket reactor (UASB) has converted anaerobic digestion into a competitive technology for high organic load wastewater (Torkian et al. 2003, Kim et al. 2004). To achieve the advantages of suspended growth and adherence growth, Lo et al. (1994) treated pig wastewater in a UASB reactor, to which a carrier medium was placed in the center of the reactor, in a temperature range between 22 and 28 °C. These authors obtained removal efficiency of COD from 95.0 % to VOL of 1.65 kg COD / m³d. However, this efficiency decreased to 57.0 % at a VOL of 3.5 kg/m³d. Campos et al. (2005), with the same configuration at 20 h HRT and VOL of 1.42 kg COD/m³d, obtained removal efficiency of COD of 84.0 %.

Pereira-Ramírez et al. (2004) used a UASB reactor with recirculation of effluent and HRT of 12 h for treating pig wastewater, varying COD/alkalinity relationship. In this study, the efficiency of COD removal of 85.0% was obtained, for an accessional speed  of 0.35 m / h with high VOL (23 kg COD/m³d), while superior speeds promoted instability in the functioning of the reactor, and caused a reduction of organic matter removal, which reached 65.0%.

Rodrigues et al. (2010), in a reactor of 11.5 m³, at HRT from 1.7 to 4.1 d, and with VOL between 1.1 and 17.5 kg COD/ m³d, obtained COD removal efficiencies of 92.0 %.

Song et al. (2010) studied different hydraulic retention times (7.0, 6.4, 5.0 and 3.5 d) in a volume of 35 m³. During the functioning period, efficiencies of COD and VFA removal were in the range from 74.0 to 78.7% and 89.3 to 96.6%, respectively. These effluent concentrations constituted an efficient removal of organic matter and stable functioning. Associated to high removal efficiency, for different HRT,  methane yield values were 71.0, 83.3, 76.9 and 71.9 %, respectively.

Results of the studies in this configuration show that this technology is suitable for the treatment of pig wastes. However, Sánchez et al. (2005) concluded that the UASB is not suitable for the treatment of pig manure, based on poor yield for low HRT and high VOL.

As previously stated, treating these wastes with UASB is effective, due to its ability of forming a granular sludge with excellent sedimentation characteristics, which allows to study in a wider range of VOL and lower HRT than in other configurations.

Apart from the different advantages that have made it successful, studies by various authors (Hickey et al. 1991, McHugh et al. 2004) show that one of the major disadvantage of UASB reactors are the long starting periods, due to the time required for the process of anaerobic granulation. Together with this fact, a good degree of mixture between biological sludge and feeding is not always achieved, resulting in insufficient mass transfer and appearance of concentration gradients. For this reason, a modified version of this configuration is produced, which is called anaerobic reactor of expanded granular sludge bed (EGSB), which operates at higher accessional speeds by recirculation of the effluent and is designed with higher height/diameter ratio, around 20 (Kato et al. 1994, O’Reilly and Colleran 2005). High accessional speed of the liquid applied to these reactors, allows a better hydraulic agitation of the sludge bed, resulting in a higher expansion of the bed and, consequently, improves biomass-substrate contact. This may reduce considerably the volume of dead zones, preferential flows and shortcuts (Nicolella 2000, Fuentes et al. 2011) and influence positively on the increase of treatment efficiency (López and Borzacconi 2011). In recent years, there has been more focus on EGSB reactors than on UASB reactors, because they endure higher organic loads, which favor their hydrodynamics (Puñal et al. 2003, Teixeira et al. 2014)

These reactors have been little studied, compared to UASB. However, due to its many advantages, they are considered as a technology with broad prospects for high-load wastes. Some research have used an EGSB reactor for treating effluents from pig breeding, mainly the studies of López-Fernández et al. (2011), Lee and Han (2012).

The first combined the EGSB reactor with ultrafiltration system and functioning at a HRT of 3.8 d and VOL of 2.76 g COD/Ld, obtained total COD removal efficiency of about 70.0 %. The second authors, using also a combination of reactors, with a batch reactor after EGSB, obtained COD removal efficiency of 42.5%, with VOL between 2.0 and 6.0 kg COD/m³d at a HRT of 24 h.

Apart from the fact that high load technologies are more complex in their functioning, they allow to obtain higher robustness to operate with lower HRT and high VOL, leading to a better cost-benefit relationship. However, studies on training and development of granules should be widened, in order to reduce startup time and improve efficiency of organic load removal with a more appropriate balance of environmental conditions inside the reactors.

ANAEROBIC TREATMENTS USED IN CUBA

For many years, breeding pig wastes were treated in conventional anaerobic biodigesters. However, this type of reactor, normally used in the treatment of these semi-solid wastes, with total solids concentrations around 6.0 %, are not suitable for the treatment of dilute wastewater, with solids concentrations of 0.1 to 3.0 %, due to the need of very high HRT and consequently, biodigesters with very high volumes.

One of the most used biodigestors, not only in Cuba but in the world, is the fixed dome. In our country there are approximately 400 plants of this type, according to the latest census conducted by the National Biogas Group (NBG) in 2015. The designs that currently exist allow adequate treatment of wastewater, up to 500 pigs (Sosa et al. 2014), because as its operation explained, they are located underground and lead to the movement of important  volumes of soil. The efficiency of the fixed dome technology reduces the contaminant load between 75.0-90.0 %, depending on the pig waste characteristics, as well as the efficiency and control of operation of the biodigester. The effluents treated in fixed dome biodigesters can be arranged in an oxidation pond and used for crop irrigation, due to their low total solids percent.

There are also, to a lesser number, the tubular biodigesters or "Plug Flow", many of them installed by researchers and specialists of the Center for the Development of Biogas (CPDB) from the Institute of Pig Research and, approximately, 120 have been installed in the Western provinces. In Cuba, these biodigesters are used in farms of up to 150 pigs, since for their correct functioning and adequate treatment of wastes , they must have a diameter/length (d/l) of 1/6 relation (Sosa et al. 2014) .

These reactors are not effective for the treatment of large volumes of wastewater (Oviedo 2011, Guardado 2013). For this reason, the technology used in Cuba for pig integral centers with a capacity higher than 500 pigs, is a piston-flow reactor with rigid cover, which allows the treatment of the generated load up to 2000 heads. These plants are capable of generating a volume of biogas that supplies the consumption of the farms where they are installed (Díaz 2012). However, due to the high HRT and the low loads, large volume reactors are needed, with the consequent low effectiveness in the treatment of these wastes.

A tendency in recent years has been the use of covered ponds which use structures and domes of geomembrane, in which the organic matter oxidation and gas retention occur. High removal efficiencies of COD are obtained in this technology, ranging from 78.0 to 90.0 %, but at high hydraulic retention times (HRT) with a minimum of 10 d (Blanco et al. 2015). On the other hand, the useful life of the geomembranes is short, so it is expensive, not only for the large volumes of ponds, but also for the cost of replacing or maintaining the geomembranes.

Although international experiences show that high-load technologies are more robust and effective from the environmental and energy point of view, in the design of treatment plants of pig wastes in Cuban industrial facilities there is no projection of the use of these technologies.

FINAL CONSIDERATIONS

Taking into account the characteristics of the pig wastes, the anaerobic digestion technology allows its degradation with high efficiency. In the world, the UASB technology is the most used in the anaerobic treatment of these wastes, because it allows the accumulation of high biomass concentrations in granular sludge form. This results in a process of high efficiency and stability, in which it is possible to work at high VOL and lower HRT than in other configurations. Regardless of this, biomass-substrate contact is still insufficient, which does not favor the hydrodynamic performance. For these reasons, the international experience shows that EGSB technology has higher advantages and is promising for the treatment of high-load wastes.

According to what has been analyzed so far, the UASB and EGSB configurations have wide potentialities of use in pig wastes treatment which are generated in facilities on an industrial scale in Cuba. However, in both cases, studies should be carried out to establish the most appropriate environmental and operational conditions, in which higher efficiencies of removal of solids and organic matter were obtained, allowing the disposal of a higher quality effluent.

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Received: 19/10/2016
Accepted: 5/12/2016

Tania Pérez- Pérez, Instituto de Ciencia Animal, Apartado Postal 24, San José de las Lajas, Mayabeque. Email: taniap@ica.co.cu