The effect of using pig manure as an internal carbon source in a traditional piggery wastewater treatment system for biological denitrification

Highlights

• Adding 1.78% PMC improved the denitrification in digested piggery wastewater.

• Effect on denitrification of adding PMC was better than adding RWC.

• The promoting effects of PMC on denitrifying bacteria were major contributor to TN removal.

Abstract

Due to a low ratio of chemical oxygen demand (COD) to total nitrogen (TN) and poor biodegradability of digested piggery wastewater, lack of a carbon source in subsequent biological treatment limits denitrification, resulting in an accumulation of NO₂-N and NO₃-N and a low denitrification efficiency. In this project, combined ultrasonication/alkali treatment of pig manure was used to generate a source of pig manure carbon source(PMC). This report mainly discusses the effects of adding the PMC on subsequent biological denitrification and microbial community changes. The experimental results showed that in subsequent biological treatment with the addition of 1.78% PMC, the effluent NO₂-N and NO₃-N were 0.02 mg·L⁻¹ and 0.90 mg·L⁻¹, respectively. The TN removal percentage was 72.29%, which was 62.29% higher than that obtained without an added carbon source and 36.74% higher than that obtained with added raw wastewater carbon (RWC). High-throughput sequencing revealed that the relative abundances of the dominant microflora Thauera and Thermomonas, which are related to denitrification, increased from 1.13% and 0.55% to 6.19% and 4.69%, respectively, after the addition of PMC. The addition of PMC improved the denitrification of digested piggery wastewater in subsequent biological treatment, and the denitrification effect was obvious and could enable waste resource utilization.

Introduction

Piggery wastewater is organic wastewater containing high concentrations of nitrogen (N) and phosphorus (P). Its direct discharge into natural water will lead to eutrophication of water bodies, endangering the aquatic environment and ecosystem and posing a serious threat to human health (Yu and Cao, 2014; Lin et al., 2016; Feng et al., 2017; Tang et al., 2018). At present, anaerobic digestion is widely used to reduce organic pollution and generate biogas for power generation (Hamelin et al., 2011). However, NH₃-N cannot be removed by anaerobic digestion, and the concentration of NH₃-N in anaerobic digestion effluent is therefore high. Due to the shortage of land resources in South China, the construction of numerous natural treatment systems for the removal of biogas slurry is not possible, and the adoption of aerobic treatment of digested piggery wastewater is necessary. Traditionally, aerobic treatment often uses a sequencing batch reactor (SBR) activated sludge process with simple and convenient operation (Deng et al., 2008; Song et al., 2011; Scaglione et al., 2013; Meng et al., 2016). However, most of the biodegradable COD is removed during anaerobic digestion (Bernet and Beline, 2009), which results in a low ratio of COD to TN (COD/TN) in anaerobic digestion effluent and poor biodegradability in subsequent treatment. In the traditional piggery wastewater A/O (anoxic/aerobic) process or SBR process, NH₃-N in the wastewater is oxidized by nitrifying bacteria to NO₂-N and NO₃-N in the aerobic tank. However, when wastewater enters the anoxic tank, there is often no carbon source in the anoxic tank, resulting in the lack of an electron donor in the denitrification process. A large amount of NO₂-N and NO₃-N cannot be degraded, resulting in a low TN removal rate. At the same time, high concentrations of NO₂-N and NO₃-N in wastewater are toxic, and direct discharge to natural water bodies can cause toxicity to aquatic organisms.

To address the lack of a carbon source in digested piggery wastewater and low denitrification, many researchers have directly added carbon sources to increase the COD/TN and enhance the denitrification process, so that NO₂-N and NO₃-N are converted to N₂, thereby reducing the NO₂-N and NO₃-N content of the effluent, improving the TN removal rate, and finally achieving nitrogen removal. Generally, two sources of carbon have been used. Direct addition of commercial carbon sources such as glucose, methanol (Jin et al., 2014), acetic acid (Obaja et al., 2003) and sodium acetate (Martin et al., 2009) has resulted in increased denitrification, but these commodity carbon sources are expensive, which limits their application. Adding carbon sources prepared from waste, such as sludge hydrolysate (Tong and Chen, 2007; Soares et al., 2010; Liu et al., 2016), hydrolytic acidification solution from food waste (Tang et al., 2019) and 30% undigested piggery wastewater (Deng et al., 2005), significantly improved the denitrification performance. However, these carbon sources are limited, and their transport is difficult. Adding 30% undigested piggery wastewater adds a large amount of NH₃-N and increases aeration but also increases the construction investment, resulting in high operating costs. Therefore, in consideration of realistic piggery conditions, it is important to find a carbon source that can be taken from local materials, is economically effective and can be added in low amounts for the biological denitrification of piggery wastewater.

Both ultrasonication and alkali treatment technologies can effectively disrupt the sludge floc structure and microbial cell walls, releasing intracellular storage polymers and soluble matter (Braeutigam et al., 2014). Related studies have shown that combining these two technologies increases SCOD dissolution (Yang et al., 2008; Kim et al., 2010; Seng et al., 2010). At present, ultrasonication/alkali pretreatment mainly focuses on the treatment of waste activated sludge (WAS) to promote anaerobic digestion to produce biogas(Chu et al., 2001; Tiehm et al., 2001; Nickel and Neis, 2007; Xie et al., 2009). However, research on ultrasonication/alkali treatment of pig manure to produce an internal carbon source and increase biological denitrification in traditional piggery wastewater has not been reported in either domestic or foreign research. Elbeshbishy et al. (Elbeshbishy et al., 2011) found that pig manure is more suitable for ultrasonication treatment than WAS and that the release effect of SCOD was improved with pig manure with the same energy. Carbon sources with increased SCOD are improve subsequent biological treatment. This indicates that the ultrasonication/alkali treatment of pig manure to produce an internal carbon source and increase biological denitrification in traditional piggery wastewater is feasible.

In this experiment, pig manure was pretreated with ultrasonication and an alkali to prepare a PMC source with a high concentration of SCOD, and the PMC source was added to digested piggery wastewater to increase denitrification. The effects of the ultrasonic energy density, ultrasonic time and pH on the release of SCOD from pig manure and the effect of PMC addition on biological denitrification and microbial community structure in subsequent biological treatment in an SBR were evaluated. The main goals were to prepare an economical and effective carbon source to increase the denitrification in digested piggery wastewater, to reduce the treatment cost of pig farm wastewater and to provide an economic, feasible and low-cost method for application to biological denitrification in piggery wastewater.