Effect of organic loading rate on anaerobic digestion of pig manure: Methane production, mass flow, reactor scale and heating scenarios

Highlights

• How organic loading rate affect anaerobic digestion of pig manure.

• The optimal organic loading rate of pig manure was 1.89 gVS/L.d(5%TS).

• Organic loading rate has significantly affected mass flow and heating requirement.

• Different heating supply scenarios have been comparatively analyzed.

• Low organic loading rate was suitable for operation in tropical regions.

Abstract

The effect of organic loading rate (OLR) with total solid (TS) control (3%–8%) on the performance of anaerobic digestion of pig manure (PM) using completely stirred anaerobic reactor was investigated. Based on the lab data, how OLR affects mass flow, construction scale and heating requirement in a farm-scale biogas plant was calculated. And three scenarios of typical reactor-heating technology were comparatively analyzed. The optimal OLR was 1.89 g volatile solid (VS)/(L^.d) with methane yield of 438.38 mL/gVS in the lab condition. The lower OLR, the larger amount of water and energy consumption, lower methane production and larger amount of liquid digestate was observed. Thus, the reactor with low OLR was suitable in tropical regions with the main target of disposing PM and fertilizer production. High OLR has advantage in the investment, but owns risk of instable process for a long-term run. In our study, among the three heating supply scenarios, biogas boiler was the best option for the designed biogas plant with the given breeding scale under moderate OLR. Combined heat and power (CHP) has potential advantage for the biogas plant under high OLR.

Introduction

In China, the manure quantities increase rapidly in a few concentrated areas as a result of large-scale and intensive livestock production moving towards (Deng et al., 2017). However, most farms do not have adequate manure management strategies to handle the large amounts of manure and prevent environment pollution (Jiang et al., 2011). In response to the problem, the Chinese government has implemented a series of management and incentive policies. All these regulations recognize that biogas technology is an environment-friendly solution to reduce environmental pollutions and produce valuable by-products, such as clean energy (biogas) and high-quality fertilizers. Besides, because of the reliability of anaerobic digestion (AD) technology and growing awareness of environmental protection from farm owners, the number of large- and medium-scaled biogas plants built in livestock farms increases with the average annual growth rate of about 9.15% from 2001 to 2006 and 21.53% from 2006 to 2010 (Yang et al., 2012). In China, nearly more than 90% of the biogas plants are built up for manure management (MOA, 2011). Particularly, China is rich in resource of pig manure (PM) with pig rearing amount of 465 million heads in 2014 (Yu et al., 2015), thereby, due to its abundance, inherent buffering and containing a wide range of nutrients for the growth of anaerobic microorganisms, PM has a great potential for scaled biogas generation (Regueiro et al., 2012). In addition, a variety of anaerobic reactors have been developed and applied in China including continuous stirred tank reactor (CSTR), up-flow anaerobic sludge blanket (UASB), up-flow solids reactor (USR), and plug flow reactor (PFR). Among which, the biogas plants with CSTR and USR technologies are prominent, comprising 65% of the total plants (Chen et al., 2012).

The AD process is complex, affected by many factors including the type and composition of the substrate, microbial composition, temperature, organic loading rate (OLR), pH and reactor configuration, etc. Many studies have reported that temperature is one of the most crucial factors influencing the microbial community, process kinetics, stability, substrate utilization rates, and methane yield (Bouallagui et al., 2009, Dela-Rubia et al., 2002, Riau et al., 2010). The AD process is commonly operated in the mesophilic temperature, considering the process stability, energy expense and microorganism sensitivity (El-Mashad et al., 2003, Fernandez et al., 2008, Ward et al., 2008).

OLR is also considered as an important operational parameter. The high OLR means high treatment capacity and methane yield, but it also may lead to overloading and thereby causing process instability, and even system failure. Li et al. (2015a) reported operating limits of OLR when livestock manure was mono-digested. Guo et al. (2013a) indicated that there might be a risk of acidification if the reactor operated below 38 °C at high OLRs. Thus, there is a need for a strategy to find a suitable OLR for achieving a stable and optimum anaerobic digestion of pig manure.

Regarding PM, its carbon to nitrogen (C/N) ratio was lower relative to the optimum C/N ratio (20:1–30:1) existing ammonium inhibition risk. Therefore, several studies, in batch or continuous experiments, have indicated the benefits of co-digestion with other wastes (Astals et al., 2013, Ren et al., 2014, Zhang et al., 2014), however, PM is still commonly used as the sole feedstock to produce biogas for most farms in China. Actually, for co-digestion, the sources, transportation, and supply stability of the co-substances should be considered to keep a long run of the biogas plant.

Some existing biogas plants have been operating at low efficiency owing to various technical barriers, such as running at lower or higher OLR, fluctuant digestion temperature, lack of knowledge about maintenance and monitoring, etc.(Deng et al., 2017). Thus, the appropriate operating conditions are essential for the commercial and sustainable development of AD technology treating PM. Most of the AD studies are in lab-scale (Guo et al., 2013a, Ni et al., 2017, Tsapekos et al., 2017) or evaluate the operational performance of existed biogas plants (Dong and Lu, 2013, Li et al., 2015b). The results and discussion from lab-scale indeed could provide insight into large-scale development. However, there are few studies linking the data from lab-scale to give design consideration for a farm-scale biogas plant, in particular, how the effects of different OLRs on the mass flow and energy requirement observed in lab-scale operation be interpreted in large-scale without building a real large-scale reactor has never been investigated. Thus, this is extremely important because it can (і) help to know the input, output and detailed mass flow and energy consumption, (іі) investigate different scenarios to provide appropriate considerations for the design of the commercial biogas plant.

In the present study, anaerobic digestion of PM was performed with different OLRs operation using total solid (TS) control (3%–8%) at mesophilic temperature (35±1 °C) in lab-scale. The data obtained was then used as input for validating the results in large-scale. The objective is to: (1) investigate how would be the effects of OLR on AD process performance in large-scale based on the simple modeling; (2) simulate the variations of mass flow, biogas plant scale and heating requirement under different OLR_S in large-scale using lab-scale data. (3) Analyze different scenarios of heat supply and sustainability.