Swine manure treatment by anaerobic membrane bioreactor with carbon, nitrogen and phosphorus recovery

Anaerobic digestion is an effective bio-process in high-strength wastewater treatment, reducing environmental hazards while simultaneously producing biogas to meet local energy needs. It has been widely applied for years to the treatment of organic waste, including animal manure. However, anaerobic digestion of animal manure has several potential drawbacks that account for its frequently poor efficiency and unsteady operation. Firstly, biodegradability of animal manure is lower than other organic wastes or energy crops (Weiland 2006). Previous studies showed that the organic matter of animal manure is incompletely degraded, a relatively high concentration of suspended solids (SS) remains in the anaerobically treated effluent and the chemical oxygen demand (COD) removal efficiency is only up to 75–80% (Nasir et al. 2012). Secondly, the high nitrogen content in animal manure leads to a high concentration of ammonia in the reactor, which may in turn inhibit anaerobic digestion (Rajagopal et al. 2013).

Anaerobic membrane bioreactors (AnMBRs) are the combination of anaerobic digestion and microfiltration/ultrafiltration membranes, thereby achieving direct solid-liquid separation by the effective membrane interception of most microorganisms present in the system. Thus, the sludge retention time (SRT) is completely separated from hydrolytic retention time (HRT), and further degradation of organic matter with lower biodegradability can be enhanced. In recent years, the feasibility and process optimization of swine manure wastewater treatment with AnMBRs have been studied, mostly in systems incorporating tubular or flat membranes (Lee et al. 2001; Padmasiri et al. 2007). A stable performance, with 90% COD removal efficiency, was obtained using an external tubular ultrafiltration membrane with the flux in the range of 5–10 L/m²/h and cross-flow velocities of up to 2 m/s. However, while the increase in cross-flow velocity benefited membrane performance, it resulted in poor anaerobic digestion (Padmasiri et al. 2007). Lee and co-workers investigated the performance of an anaerobic reactor with a submerged membrane module in the treatment of swine manure wastewater, reporting a COD removal efficiency of 80% and a methane yield of 0.32 m³/kgCOD_removed (Lee et al. 2001). In that study, membrane fouling was controlled using a stainless-steel pre-filter and aeration cleaning, both of which contributed to the reactor's long period of steady operation. Recently, an expanded granular sludge bed with a submerged hollow-fiber membrane enabled the effective treatment of swine wastewater was reported, with a COD removal efficiency of 90% (Lopez-Fernandez et al. 2011). Moreover, the feasibility of using the hollow-fiber membrane in an anaerobic reactor was demonstrated by the successful treatment of other wastewaters characterized by a high concentration of solids, e.g., excess sludge (Xu et al. 2011; Dagnew et al. 2013). However, the membrane fouling mechanism and long-term operation strategy of AnMBRs with hollow-fiber membrane for swine manure treatment were not fully investigated. Besides, although the AnMBR process could intercept solids and improve the organic loading, the permeate still contains a certain amount of ammonia and phosphorus, and needs further treatment.

Here, in this study, we investigated the feasibility of treating swine manure wastewater in an AnMBR, and recovery of ammonia, phosphorus in the permeate for carbon, nitrogen and phosphorus recycling. The treatment efficiency, methane yield and optimal pH for struvite formation were determined. Our discussion includes an analysis of the strategies to control membrane fouling, a comparison of the performance and bacterial community structures between the AnMBR and continuous stirred tank reactor (CSTR) systems, and an evaluation of the role of the membrane in the anaerobic digestion process.

The swine manure used in this experiment was collected from a pig farm on Chongming Island, Shanghai, and stored at 0–4 °C before use. The raw swine manure was diluted ten-fold with tap water to simulate the typical wastewater washing procedure on the pig farm. Table 1 shows the characteristics of the swine manure wastewater, which is a typical high strength wastewater contained high concentrations of COD, SS and nitrogen. The seed sludge used in this study was mesophilic anaerobic granular sludge taken from an internal circulation reactor of a paper mill plant in Jiangsu, China. The concentration of the total suspended solids (TSS) and volatile suspended solids (VSS) in the seed sludge were 56.2 g/L and 45.7 g/L, respectively. The sludge was inoculated without pre-treatment.

Table 1

Characteristics of the inflow

Parametera .	Value .	Parametera .	Value .
pH	7.6 ± 0.1	SCOD	3,705 ± 240
Total solids	10,850 ± 2,640	TN	743 ± 15
VS	7,945 ± 2,156		250 ± 57
SS	10,206 ± 2,902	TP	–
VSS	7,521 ± 2,313	SOP	98 ± 13
TCOD	13,476 ± 2,238	VFA	1,722 ± 415

Parametera .	Value .	Parametera .	Value .
pH	7.6 ± 0.1	SCOD	3,705 ± 240
Total solids	10,850 ± 2,640	TN	743 ± 15
VS	7,945 ± 2,156		250 ± 57
SS	10,206 ± 2,902	TP	–
VSS	7,521 ± 2,313	SOP	98 ± 13
TCOD	13,476 ± 2,238	VFA	1,722 ± 415

VS, volatile solids; SS, suspended solids; VSS, volatile suspended solids, TCOD, total chemical oxygen demand; SCOD, soluble chemical oxygen demand, TN, total nitrogen; TP, total phosphorus, SOP, soluble orthophosphate; VFA, volatile fatty acids.

^aExcept for pH, the units of all the other parameters are mg/L.

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Initially, the performance of the anaerobic CSTR without the membrane module in the treatment of swine manure was investigated with respect to biogas production and ammonia nitrogen accumulation. Table 2 lists the operational parameters for the AnMBR. Since the swine manure had a high SS concentration, it was difficult to pump it into the reactor continuously in our study. The AnMBR was operated under semi-continuous condition without pH control. The permeate/effluent was discharged once a day, and then fresh swine manure was added into the reactor. A recycling sludge pump was used for continuous sludge recycling between the CSTR and the AnMBR to ensure that a stable mixed liquor suspended solids (MLSS) concentration of 32.3 ± 6.2 g/L was reached. One hundred mL of sludge sample was removed daily, and 1–2 L of sludge was discharged after each physical cleaning of the system.

Ammonia and phosphorus in the permeate of the AnMBR was recovered as struvite by crystallization, and jar test was conducted for optimal conditions. The crystallization of the permeate was evaluated in the 1,000 mL beakers with 800 mL permeate at three pH (8, 9 and 10). The permeate was magnetically stirred with a rotating speed of 400 r/min for 30 min right after adjusting pH with 2 mM NaOH. The precipitate was filtered out and dried for 48 hours at 40 °C and then analysed for its weight and components. The ion concentrations (soluble orthophosphate (SOP), ⁠, Mg²⁺, Ca²⁺) of the filtrate was also analyzed for the calculation of recovery efficiency.

The liquid samples were centrifuged at 10,000 rpm for 10 min and filtered immediately through 0.45-μm filters. TSS, VSS and SOP were analyzed in accordance with APHA Standard Methods (APHA 1998). Total and soluble chemical oxygen demand (TCOD and SCOD), total phosphorus (TP), total nitrogen (TN) and ammonia nitrogen were determined using a DRB200 and a DR2800 digital reactor block and the method described by the manufacturer (Hach Co., Colorado, USA). The pH value was detected using a EUTECH-510 gas electrode (California, USA).

Extracellular polymeric substances (EPS) were extracted from the sludge by thermal treatment (Wang et al. 2006) and soluble microbial products (SMP) were extracted from the liquid sample after filtration through 0.22-μm filters. The polysaccharide and protein contents of the EPS and SMP were analyzed using the phenol-sulfuric acid method (calibrated against glucose) and Folin-Ciocalteu method (calibrated against bovine serum albumin), respectively. Volatile fatty acids (VFAs, C2–C5), biogas production, and biogas composition were determined as described in our previous publication (Wang et al. 2011). Metal ions were determined using Ion Chromatography (Dionex ICS-3000, Sunnyvale, CA, USA) as described in (Xie et al. 2012). The precipitate was analysed by X-ray diffraction scanning (XRD, RigakuD/max), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS, Philips XL30 D6716).

High performance size exclusion chromatography (HPSEC) was employed to determine the apparent molecular weight distribution of the inflow, fermentation liquor and permeate. A gel filtration chromatography analyzer was used, which consisted of a TSK gel 4000SW column (TOSOH Corporation, Japan) and a liquid chromatography spectrometer (LC-10AD, Shimadzu, Japan). Polyethylene glycol (Merck Corporation, Germany) standards with different molecular weight were used for calibration. The elution at different time intervals was collected by an automatic fraction collector, and then analyzed automatically via a differential detector to obtain the molecular weight distribution.

The microbial community was collected from the sludge by centrifugation, and the analytical method was described in our previous publication (Wang et al. 2011). The primers used in this study were F340-GC (5′-CGC CCG CCG CGC GCG GCG GGC GGG GCG GGG GCA CGG GGG G CCC TAC GGG GYG CAS CAG-3′) and R519 (5′-TTACCG CGG CKG CTG-3′). The ∼250-bp fragment of the V3 region of 16S rDNA was amplified in a polymerase chain reaction (PCR) using the primers. Sequence similarity searches were performed using the Basic Local Alignment Search Tool (BLAST). The results were used to search the National Center for Biotechnology Information sequence database (http://www.ncbi.nlm.nih.gov/BLAST/).

The effluent quality and treatment efficiency of the CSTR were also compared to that of the AnMBR (Table 3). As shown, the COD concentration of the AnMBR effluent was much lower than that of the CSTR effluent. The permeate contained negligible amounts of TSS and VFAs, whereas the effluent from the CSTR (fermentation liquor) still contained 10.1 ± 0.7 g TSS /L and 30 mg VFAs /L. The above results indicated that the membrane module achieved effective interception and enhanced further degradation of organic matter, and the latter led to a methane yield of the AnMBR 83% higher than that of the CSTR. In addition, the ammonia nitrogen content of the permeate was similar to that of the fermentation liquor (495 and 485 mg/L, respectively); while the average SOP concentration was much lower (27.4 ± 9.0 and 51.8 ± 9.0 mg/L, respectively). These results further confirmed that interception by the membrane module played an important role in the AnMBR system.

Table 3

Comparison of effluent quality and treatment efficiency between the CSTR and AnMBR system

.	Parameter .	CSTR .	AnMBR .
Effluent quality	COD (mg/L)	8,643 ± 841	491 ± 112
	SS (g/L)	10.1 ± 0.7	0
	VFA (mg/L)	30.4 ± 3.5	0
	(mg/L)	485 ± 43	495 ± 32
	SOP (mg/L)	51.8 ± 9.3	27.4 ± 9.0
Treatment efficiency	COD removal (%)	38 ± 6.9	96 ± 1.1
	Methane yield (mL/gVSadded)	153 ± 64	280 ± 93

.	Parameter .	CSTR .	AnMBR .
Effluent quality	COD (mg/L)	8,643 ± 841	491 ± 112
	SS (g/L)	10.1 ± 0.7	0
	VFA (mg/L)	30.4 ± 3.5	0
	(mg/L)	485 ± 43	495 ± 32
	SOP (mg/L)	51.8 ± 9.3	27.4 ± 9.0
Treatment efficiency	COD removal (%)	38 ± 6.9	96 ± 1.1
	Methane yield (mL/gVSadded)	153 ± 64	280 ± 93

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To investigate how the addition of the membrane module affected the microorganisms within the reactor, the structures of the bacterial communities in the AnMBR system and CSTR system were analyzed. When the operational conditions in the two reactor systems had reached steady-state, the bacterial community was sampled and its structure subsequently analyzed by PCR-denaturing gradient gel electrophoresis (DGGE).

By contrast, according to the DGGE sequencing results, there was little different in the archaebacteria (methanogens) present in the two systems. As listed in Table 4, three bands (2, 10, and 14) corresponding to methanogens were obtained in the AnMBR. These were subsequently identified as Methanobacterium palustre, Methanoculleus marisnigri and Methanosaeta concilii. Those species together with Methanosaeta thermophile (band 9) were also detected in the CSTR. The slightly lower microbial diversity in the AnMBR suggests that M. thermophile was unable to adapt to the operational conditions of the system such that it gradually died out (Chen et al. 2009).

When the permeation/relaxation rate was decreased from 10/1 to 5/1 (stage 5 to stage 6), the steady operation time of the membrane module improved from 15 days to 20 days and the fouling rate was reduced by about 10%. Besides, the TMP increase rate in stage 6 was slower than stage 5 (Figure 5), which indicated the decreasing permeation/relaxation rate had a significant effect on slowing down membrane fouling. However, in terms of handling capacity, a decrease in the permeation/relaxation rate reduced the filtration efficiency by 8.3%. Thus, practical applications should consider the influent volume if the membrane fouling rate is controlled by adjusting the permeation/relaxation rate.

After 100 days of reactor operation, the filtration efficiency of the membrane module was recovered by chemical cleaning as follows: the membrane surface was washed with water to remove the adherent sludge, after which it was immersed first in NaClO (200 mg/L) and then in citric acid (125 mg/L) for 48 h, respectively. After a second wash with water, the membrane was immersed in water until the alkalinity of the water was >70 mg/L. After chemical cleaning, the membrane was operated steadily for 20 days and exhibited both a higher flux and a 60% decrease in the fouling rate compared to the first stage under the same operation conditions. This result indicated that the filtration capacity could be fully recovered by chemical cleaning in the early phase of membrane fouling, which was a very important reference for long term operation of the membrane module. A certain frequency of on-line chemical cleaning is very essential for long period operation, and the exact frequency and methods should be determined by the cleaning effect, cost, the influence on anaerobic degradation and on-site situations etc.

In this study, certain amounts of precipitates were observed in the bottom of the permeate collecting tank. And the average concentrations of and of the collected permeate were 11 mg/L and 13.8 mg/L lower than those of the fermentation liquor, respectively. Considering the pH of the permeate was 7.9 ± 0.1, and contained magnesium (61.6 ± 7.8 mg/L) and calcium (47.7 ± 3.3 mg/L), ammonia and phosphorus precipitation might occur in the permeate. Therefore, recovery of ammonia and phosphorus from the permeate under different pH (8, 9, 10) were examined in this study.

In this work, the feasibility of treating swine manure wastewater in an AnMBR and the recovery of ammonia and phosphorus in the permeate were investigated. The AnMBR system achieved steady operation under a high concentration of MLSS for 120 days, with a total COD removal efficiency of 96% and methane yield of 280 mL/gVS_added. The methane yield of the AnMBR is 83% higher than that of the single CSTR, which indicates further degradation of organic matters in the AnMBR system. Moreover, the relationship between membrane fouling rate and MLSS, EPS and SMP, and the effects of permeation/relaxation rate and physical, chemical cleaning on membrane fouling were discussed in this study. MLSS and the polysaccharide contents of the EPS were linearly correlated with membrane fouling rate, and thus considered to be the direct causes of membrane fouling. And a decrease in the permeation/relaxation rate together with chemical cleaning effectively reduced membrane fouling. Ammonia and phosphorus was recovered as struvite by crystallization at pH 8, 9 and 10, of which pH 9 was found to be optimum for struvite formation. The study established a two-step treatment procedure that combined the AnMBR and crystallization process for nutrient recovery from swine manure wastewater, which has a profound significance for resource recovery and utilization of high strength wastewater.

This research was supported by GE CTC, National Natural Science Foundation of China (51178326, 51378373), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCCRE16015), and Fundamental Research Funds for the Central Universities.