Treatment and utilization of swine wastewater - A review on technologies in full-scale application.

The management of swine wastewater has become the focus of attention in the farming industry. The disposal mode of swine wastewater can be classified as field application of treated waste and treatment to meet discharge standards. The status of investigation and application of unit technology in treatment and utilization such as solid-liquid separation, aerobic treatment, anaerobic treatment, digestate utilization, natural treatment, anaerobic-aerobic combined treatment, advanced treatment, are reviewed from the full-scale application perspective. The technologies of anaerobic digestion-land application is most appropriate for small and medium-sized pig farms or large pig farms with enough land around for digestate application. The process of "solid-liquid separation-anaerobic-aerobic-advanced treatment" to meet the discharge standard is most suitable for large and extra-large pig farms without enough land. Poor operation of anaerobic digestion unit in winter, hard to completely utilize liquid digestate and high treatment cost of digested effluent for meeting discharge standard are established as the main difficulties.

Effect of air mixing on high-solids anaerobic digestion of cow manure: Performance and mechanism.

This study aimed to further investigate the effect of air mixing on the performance of a high-solids anaerobic digestion system and reveal its underlying mechanisms via analyses of carbon conversion, microbial communities and key functional genes. When the air mixing intensity was 12.5, 37.5 and 62.5 mL/(L‧min), compared with the anaerobic digestion without air mixing, the methane yield was increased by 6 %, 13 % and 6 %, respectively. The improved performance was partly attributed to the increased hydrolysis rate of macromolecular substances by 5 %-16 % and carbon recovery in the form of methane by 6 %-7% compared with the controls. Functional flora (Magnetospirillum, Synergistaceae) and hydrolytic metabolism-related enzymes (cellulose, α-amylase) demonstrated higher abundance under air mixing condition, thus promoting the degradation of organic matter and methane production. This work provides some new insights into the use of air mixing to improve anaerobic digestion of high-solids waste.

Promotion of nitrogen removal in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode.

In this study, the performance and mechanism of nitrogen removal were investigated in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode with sodium acetate as the organic carbon source. The results showed that the additional organic matter had the capacity to promote NH4+-N and total inorganic nitrogen (TIN) removal with efficiencies of 91.09% and 84.10%, and increases of 60.06% and 75.32% compared with the control group, respectively. The organic matter also stimulated the production of extracellular polymer substances that reduced the passivation and toxicity of iron to microorganisms. The ammonia oxidation activity was 2.5 times higher than that in the control group, and the anammonia oxidation activity and denitrification activity were substantially higher than in the control group with TIN removal efficiencies of 1.02 and 1.19 mgN/(gVSS·d), respectively. In addition, the organic matter increased the enrichment of the heterotrophic denitrification bacterium Diaphorobacter and facultative iron salt-based bacterium Dechloromonas.

Intermittent air mixing system for anaerobic digestion of animal wastewater: Operating conditions and full-scale validation.

An air mixing system for anaerobic digestion has been proved to be beneficial for methane production. The aim of the present study was to further investigate the appropriate conditions for air mixing. The effective methane production time (EMPT) was defined to determine the air mixing time in the article. The results indicated that the appropriate aeration intensity was 66.7 mL air per volume of reactor per min and mixing time was 1.5 min. When air mixing time exceeded 3 min on each occasion, total CH4 production was less than that achieved under the no mixing condition due to a decrease in the EMPT. In addition, the possibility of air mixing was evaluated in an anaerobic full-scale plant comprising a continuous stirred tank reactor. One year of operating data validated the feasibility of air mixing during the anaerobic digestion of swine wastewater.

Construction of autotrophic nitrogen removal system based on zero-valent iron (ZVI): performance and mechanism.

In this study, the performance and mechanism of nitrogen removal in sequencing batch reactors (SBRs) with and without zero-valent iron (ZVI) was investigated. The results showed that ZVI had a capacity to promote NH4+-N conversion, NO2--N accumulation and total inorganic nitrogen (TIN) removal, with the TIN removal rate being increased by 29.45%. The ZVI also had a significant impact on microbial community structure by means of high-throughput pyrosequencing, increasing the enrichment of Anammox (anaerobic ammonium oxidation) bacteria Candidatus Brocadia and Feammox (anaerobic ferric ammonium oxidation) bacteria Ignavibacterium. With ZVI addition, the main pathway of nitrogen removal was changed from nitrification-heterotrophic denitrification to Anammox and Feammox.

Theoretical Approach for the Calculation of the Pressure Drop in a Multibranch Horizontal Well with Variable Mass Transfer.

In this study, the pressure drop obtained from physical experiments and theoretical approaches of a single horizontal wellbore is reviewed and a comprehensive wellbore pressure-drop model is derived for a multibranch well. We propose a new coupling model for fluid flow in multibranch wells and reservoirs. Based on this coupling model, we introduce a theoretical approach for the calculation of the pressure drop in a multibranch horizontal well with variable mass transfer. To facilitate the understanding of the physical model, the entire coupling model was divided into three parts: (1) the pressure-drop model of the wellbore, (2) the reservoir inflow model, and (3) the coupling model. By incorporating the acceleration, friction, mixing, confluence, and gravity pressure drops, a coupling model with a finite-conductivity multibranch horizontal well was developed. Newton-Raphson iterations and Visual Basic programming were employed to solve the coupling model and to obtain the pressure and the inflow rate of the wellbore. The wellbore pressure-drop model was verified by comparing it with different models for the same case study, which has been previously introduced in a different research work. Furthermore, the forecast and sensitivity analysis were conducted, and then the results are discussed. In the proposed new model, several factors are considered, including the wellbore structure, the wellbore completion method, the wellbore, and the fluids and formation properties. The presented approach can be used as a valuable tool to analyze the influence of the pressure drop on the productivity of complex-structured wells and vice versa, and to quantitatively investigate the various pressure drops in wellbores, including the friction, acceleration, mixing, confluence, and gravity pressure losses.

Using air instead of biogas for mixing and its effect on anaerobic digestion of animal wastewater with high suspended solids.

This study proposed a new mixing method for anaerobic digestion treating animal wastewater using air as gas source of agitation (named "air mixing") and demonstrated its feasibility by comparing with other mixing modes. The results indicated that the methane production for air mixing was increased by 6.4%, 11.9% and 19.6% compared with biogas mixing, mechanical mixing and no mixing. Air mixing improved the mass transfer and the homogeneous mixing time was shortened from 10 min of mechanical mixing to 1.5 min at the same power input. A transient microaerobic environment was created by air mixing, which increased the hydrolysis efficiency by 1.7-11.4% compared with biogas mixing and facilitated VFAs generation and consumption, as well as promoted the syntrophic relationship between facultative bacteria and hydrogenotrophic methanogens. The relative contribution of the improvement of mass transfer and the reaction of microaerobic environment to methane production was 62.9% and 37.1%, respectively.

Improved biogas production of dry anaerobic digestion of swine manure.

In this work, wrapped granular activated carbon (GAC) and acclimated sludge were employed to enhance the efficiency of the dry anaerobic digestion of swine manure in semi-continuous tests. The addition of wrapped GAC increased the volumetric biogas production rate by 10.6%, and the removal efficiencies of TS and VS were enhanced by 5.3% and 6.6%, respectively. The concentration of total volatile fatty acids (TVFA) was 30.3% lower in the GAC reactor, but the total ammonia nitrogen (TAN) content was 15.3% higher. Inoculating with acclimated sludge helped the system survive unfavorable conditions, where the TAN and TVFA contents were around 5,200 mg/L and 8,800 mg/L, respectively, but it failed to improve the biogas production efficiency. The mechanisms that allowed GAC to increase the production of biogas were related to the improved hydrolysis process, enhanced microbial adhesion, the provision of electronic bridges, and enrichment of functional microorganism.

[Effect of Different Temperatures on the Performance of Autotrophic Nitrogen Removal and Microbial Community from Swine Wastewater].

Four laboratory-scale autotrophic nitrogen removal bioreactors were implemented to investigate performance differences and microbial mechanisms under different temperatures (30, 25, 20, and 15℃). The results showed that the reactor performance under 30℃ was higher than others. When the temperature decreased from 30℃ to 25℃, total nitrogen removal efficiency reduced from 73% to 66%, and total nitrogen removal rate from 2.29 kg·(m3·d)-1 to 1.72 kg·(m3·d)-1. The morphology and particle size of the sludge did not change significantly (SMD:from 80.85 µm to 79.95 µm). When the temperature was 20℃, the total nitrogen removal efficiency reduced to 42%, the total nitrogen removal rate reduced to 1.18 kg·(m3·d)-1, and the sludge disintegration phenomenon occurred with particle size reduction (SMD:63.21 µm). When the temperature was 15℃, the total nitrogen removal efficiency reduced to 37%, and the total nitrogen removal rate got as low as 1.00 kg·(m3·d)-1. In addition to that, the reactor operation was difficult. The analysis of microbial community structure showed that the influence of temperature on anaerobic ammonia oxidizing bacteria is greater than that on ammonia oxidizing bacteria. This sensitivity to temperature of the anaerobic ammonia oxidizing bacteria was the main reason for the decreased performance under low temperature conditions.

Control of partial nitrification using pulse aeration for treating digested effluent of swine wastewater.

Three sequencing batch reactors (SBRs) were used to investigate the influence of pulse frequencies on the partial nitrification (PN) process in this study. At a total aeration time of 6 min each hour, the aerated frequencies of R1, R2 and R3 were 6, 3 and 2 time h-1. During the steady period (117-143d), the nitrite accumulation rates (NARs) were 90.80%, 90.71% and 90.23% in R1, R2 and R3, respectively, indicating a steady nitritation was acquired. Activity measurements of the sludge samples taken at day 138 showed the activity of nitrite oxidating bacteria (NOB) was 0, indicating NOBs were successfully suppressed. The ratio of NO2--N to NH4+-N in the effluent of R3 was 1.35, which most closely matched the influent of Anammox process. However, the energy efficiency evaluation showed that R1 had the highest actual oxygen transfer efficiency (AOTE) and dynamical efficiency (DE).

Two-step heating mode with the same energy consumption as conventional heating for enhancing methane production during anaerobic digestion of swine wastewater.

With high percentage of washing water, swine wastewater is characterized by large volume and low concentration of total solids. Thus, in treating swine wastewater, it is relatively difficult to heat digesters, resulting in low methane production at low ambient temperatures (ATs). To increase methane production from swine wastewater, this study proposed a novel "two-step heating (TSH)" mode with the same energy consumption as a one-step process for anaerobic digestion. Compared with the traditionally heated digesters (one-step heating), the digestion temperature in TSH digesters increased by 3.50-10.50 °C under the assumption of no heat dissipation and by 3.30-9.25 °C in the actual experiments. Although methane production of the TSH digesters improved by 15% in our experiments, the improvement was far less than theoretically estimated. This was mainly caused by short hydraulic retention time and sludge washout in the digesters. Moreover, the acetoclastic methanogenesis, accomplished by genus Methanosaeta, was the major methanogenesis pathway at low temperatures in both the TSH and conventional heating modes. However, the relative abundance of syntrophic bacteria and hydrogenotrophic methanogens in TSH mode were both higher than in the digesters operation in conventional heating mode when the atmospheric temperature was below 10 °C.

Performance of autotrophic nitrogen removal from digested piggery wastewater.

The performance of an autotrophic nitrogen removal process to treat digested piggery wastewater (DPW) was investigated by gradually shortening the HRT and enhancing the DPW concentration during 390days of operation. The results showed that the total nitrogen removal rate and efficiency reached 3.9kg-Nm-3day-1 and 73%, which were significantly higher than the levels reported previously. A high relative abundance of Nitrosomonas (4.2%) and functional microbes (12.15%) resulted in a high aerobic ammonium oxidizing activity (1.25±0.1g-NgVSS-1d-1), and a good settling ability (SVI, 78.42mLg-1SS) resulted in a high sludge concentration (VSS, 11.01gL-1), which laid a solid foundation for the excellent performance. High-throughput pyrosequencing indicated that, compared with synthetic wastewater, the DPW decreased the relative abundances of every functional group of nitrogen removal microbes, and increased relative abundances of anaerobes (15.7%), sulfur-oxidizing bacteria (9.4%) and methanogens (40.8%).

The Enrichment of Microbial Community for Accumulating Polyhydroxyalkanoates Using Propionate-Rich Waste.

Polyhydroxyalkanoates (PHAs) are promising alternatives to plastics since they have similar properties to polyolefin but are biodegradable and biocompatible. Recently, the conversion of propionate wastewater to PHAs by undefined mixed microbial cultures becomes attractive. However, how microbial community changes remains unclear during the enrichment step, which is critical for a robust PHA-producing system. In this study, PHA-accumulating cultures were enriched under feast/famine condition using propionate-rich substrates. Our results showed that during the first 2 h of the enrichment, dissolved oxygen of cultures increased remarkably until saturation, and amounts of C, N, and chemical oxygen demand of cultures decreased significantly to a very low level. High-throughput sequencing revealed that bacterial populations affiliated with Alphaproteobacteria and Bacteroidetes dominated the cultures enriched. Most of these dominant populations contributed to the conversion of short-chain fatty acids to PHAs. Being fed with the substrate rich in propionate but without nitrogen, the cultures enriched could accumulate nearly 27% PHAs at 72 h with higher content of hydroxyvalerate. Our work reveals the process in which environmental microbes responded to propionate-rich condition and shifted to populations for accumulating PHAs; it also will be helpful to develop an efficient PHA-producing system using propionate-rich waste.

A model for methane production in anaerobic digestion of swine wastewater.

A study was conducted using a laboratory-scale anaerobic sequencing batch digester to investigate the quantitative influence of organic loading rates (OLRs) on the methane production rate during digestion of swine wastewater at temperatures between 15 °C and 35 °C. The volumetric production rate of methane (Rp) at different OLRs and temperatures was obtained. The maximum volumetric methane production rates (Rpmax) were 0.136, 0.796, 1.294, 1.527 and 1.952 LCH4 L(-1) d(-1) at corresponding organic loading rates of 1.2, 3.6, 5.6, 5.6 and 7.2 g volatile solids L(-1) d(-1), respectively, which occurred at 15, 20, 25, 30 and 35 °C, respectively. A new model was developed to describe the quantitative relationship between Rp and OLR. In addition to the maximum volumetric methane production rate (Rpmax) and the half-saturation constant (KLR) commonly used in previous models such as the modified Stover-Kincannon model and Deng model, the new model introduced a new index (KD) that denoted the speed of volumetric methane production rate approaching the maximum as a function of temperature. The new model more satisfactorily described the influence of OLR on the rate of methane production than other models as confirmed by higher determination coefficients (R(2)) (0.9717-0.9900) and lower bias between the experimental and predicted data in terms of the root mean square error and the Akaike Information Criterion. Data from other published research also validated the applicability and generality of the new kinetic model to different types of wastewater.

Effect of temperature on continuous dry fermentation of swine manure.

Laboratory-scale experiments were performed on the dry digestion of solid swine manure in a semi-continuous mode using 4.5 L down plug-flow anaerobic reactors with an organic loading rate of 3.46 kg volatile solids (VS) m(-3) d(-1) to evaluate the effects of temperature (15, 25 and 35 °C). At 15 °C, biogas production was the poorest due to organic overload and acidification, with a methane yield of 0.036 L CH4 g(-1) VS added and a volumetric methane production rate of 0.125 L CH4 L(-1) d(-1). The methane yield and volumetric methane production rate at 25 °C (0.226 L CH4 g(-1) VS added and 0.783 L CH4 L(-1) d(-1), respectively) were 6.24 times higher than those at 15 °C. However, the methane yield (0.237 L CH4 g(-1) VS added) and the volumetric methane production rate (0.821 L CH4 L(-1) d(-1)) at 35 °C were only 4.86% higher than those at 25 °C, which indicated similar results were obtained at 25 °C and 35 °C. The lower biogas production at 35 °C in dry digestion compared with that in wet digestion could be attributed to ammonia inhibition. For a single pig farm, digestion of solid manure is accomplished in small-scale domestic or small-farm bioreactors, for which operating temperatures of 35 °C are sometimes difficult to achieve. Considering biogas production, ammonia inhibition and net energy recovery, an optimum temperature for dry digestion of solid swine manure is 25 °C.

Co-digestion of tobacco waste with different agricultural biomass feedstocks and the inhibition of tobacco viruses by anaerobic digestion.

Tobacco is widely planted across the world especially in China, which means that a large amount of tobacco waste needs to be treated. This study investigated the biogas fermentation of tobacco stalks co-digested with different biomass feedstocks and the inactivation of Tobacco mosaic virus (TMV), Cucumber mosaic virus (CMV) by anaerobic digestion. Results showed that the maximum methane yield of tobacco stalks at 35 °C was 0.163 m(3) CH4 ⋅ kg VS(-1), which was from the co-digestion of tobacco stalks, wheat stalks and pig manure. The largest VS removal rate of tobacco stalks was 59.10%. Proven by indicator paper stripe, half-leaf lesion and RT-PCR, CMV could be inactivated by mesophilic and thermophilic anaerobic digestion, whereas TMV could be only inactivated by thermophilic anaerobic digestion over 20 days. These results suggested that using tobacco stalks as feedstock for anaerobic digestion and applying the digested residue and slurry to Solanaceae crop land are feasible.