Ammonia removal from chicken manure digestate through vapor pressure membrane contactor (VPMC) and phytoremediation

Highlights

• Ammonia removal of 93.6% was achieved from chicken manure digestate through VPMC.

• Mass transfer of ammonia was not affected from the permeate acid concentration.

• Separation factor of ammonia decreased with increased permeate acid concentration.

• Total ammonia concentration was decreased below 2 mg/L through integrated system.

Abstract

Ammonia removal from synthetic ammonia solutions and chicken manure digestate via vapor pressure membrane contactor through Polytetrafluoroethylene (PTFE) membrane was investigated. The highest ammonia mass flux, separation factor, and removal efficiencies of 28.6 ± 0.2 g N/m² h, 53.9 ± 10.7, and 97.6 ± 0.7% were observed for synthetic solutions, respectively. Ammonia removal efficiency of 93.6 ± 1.9% through membrane contactor was observed for chicken manure digestate decreasing the total ammonia concentration from 3643.5 ± 67.2 to 230.9 ± 46.2 mg N/L. Phytoremediation via Lemna minor species was used as a polishing step to remove remaining ammonia from the membrane contactor effluent. Total ammonia concentration was then decreased below 2 mg N/L through evaporation, nitrification, and plant uptake processes occurring in the phytoremediation containers. This study reveals that ammonia can be successfully removed via VPMC and phytoremediation systems and the process is implementable as it can be coupled to anaerobic digestion processes to recover ammonia and to prevent ammonia inhibition.

Introduction

Increasing chicken meat demand results in intensive chicken farming and thus high production of chicken manure is reported worldwide (Nie et al., 2015). Chicken manure has high nutrient content and has been traditionally used as a soil fertilizer, however application of chicken manure to soil in excess amounts may be the significant source of air and/or water pollution (Holm-Nielsen et al., 2009, Nie et al., 2015). Anaerobic digestion of animal manure such as chicken manure is an effective treatment option, which results in the production of valuable products such as biogas and digestate (Anjum et al., 2017, Holm-Nielsen et al., 2009, Kelleher et al., 2002, Nie et al., 2015). Biogas, with its methane content, is converted to electricity and heat whereas digestate, with its high nutrient content, is utilized as a fertilizer (Holm-Nielsen et al., 2009). However, during anaerobic digestion process of protein-rich wastes such as chicken manure; proteins, amino acids and uric acid, are converted to ammonia (Angelidaki and Ahring, 1994, Kelleher et al., 2002, Krakat et al., 2017, Salminen and Rintala, 2002, Singh et al., 2010). Total ammonia in water, can present as free ammonia (NH₃) and/or ionized ammonia (NH₄⁺) depending on the pH and temperature of the solution (Anthonisen et al., 1976). Free ammonia fractions is known to be responsible of the ammonia inhibition in anaerobic reactors (Angelidaki and Ahring, 1994) and therefore, ammonia inhibition is generally the biggest concern during anaerobic digestion of chicken manure (Kelleher et al., 2002, Liu et al., 2012, Nie et al., 2015). In order to prevent ammonia inhibition, most common methods applied are; pH and operational temperature control (Yenigün and Demirel, 2013), waste dilution (Kayhanian, 1999, Kelleher et al., 2002), feedstock C/N ratio adjustment through co-digestion with another waste (Bayrakdar et al., 2017a, Hassan et al., 2016, Kayhanian, 1999), enrichment of ammonia tolerant biomass (Fotidis et al., 2017), ammonia separation through membrane processes (Bayrakdar et al., 2017b, Lauterbock et al., 2012), ion exchange (Almutairi and Weatherley, 2015), and ammonia stripping (Yao et al., 2017).

Among all the listed methods, enrichment of ammonia tolerant bacteria is beneficial as it provides improved biogas production without changing the operational parameters and/or infrastructure to control ammonia inhibition (Fotidis et al., 2017). Acclimation of bacteria to high ammonia concentrations through stepwise increase of ammonia concentrations is proven to be effective in terms of operating anaerobic digesters at ammonia concentrations higher than the reported inhibitory levels (Bayrakdar et al., 2017a, Fotidis et al., 2017). However, acclimation of bacteria to high ammonia concentrations results in production of digestate with ammonia concentrations greater than 5000 mg/L (Bayrakdar et al., 2017a, Fotidis et al., 2017). In order to decrease the ammonia concentration of the digestate; ammonia needs to be separated/recovered from the digestate. Ammonia separation/recovery has been successfully achieved through tubular PTFE membrane contactors (Ahn et al., 2011), flat sheet or hollow fiber membrane contactors (Hasanoğlu et al., 2010, Lauterbock et al., 2012), Polyvinylidene fluoride (PVDF) hollow fiber membrane contactors (Tan et al., 2006), or by sweep gas membrane distillation (Xie et al., 2009). Generally, the membrane separation processes are pressure driven, electrically driven, or vapor pressure difference/concentration gradient driven (Aydin et al., 2018). Ammonia removal from synthetic ammonia solutions (Ashrafizadeh and Khorasani, 2010, Hasanoğlu et al., 2010), swine manure (Garcia-Gonzalez et al., 2015, Vanotti et al., 2017), or digestate of agricultural residuals (Lauterbock et al., 2012) has been generally achieved through microporous tubular or hollow fiber membrane contactors employing vapor pressure difference as a driving force (Tan et al., 2006). In addition, vacuum membrane distillation (El-Bourawi et al., 2007) and sweep gas membrane distillation (Xie et al., 2009) were used in order to remove ammonia from synthetic ammonia solutions. Commonly, vapor pressure difference/concentration gradient driven processes such as vapor pressure membrane contactors (VPMC) are used for the separation of volatile species such as cyanine, volatile fatty acids, and ammonia from water. VPMC systems use microporous membranes, which are also commonly used as microfiltration membranes. Nevertheless, the separation principle in VPMC systems is completely different from a pressure driven systems such as microfiltration, as the separation does not base on size exclusion and there is no convective flow (Aydin et al., 2018). Consequently, volatile species or gaseous compounds diffuse through the air filled microporous hydrophobic membranes of VPMC systems and the separation principle is based on concentration gradient or vapor pressure difference on two sides of the membrane (Albrecht et al., 2005, Aydin et al., 2018, Han et al., 2005, Lauterbock et al., 2012). In isothermal batch operations, volatile species such as ammonia, hydrogen cyanide, or volatile fatty acids diffuse through the membrane until the feed side and permeate side concentrations are equal to each other (Ahn et al., 2011, Aydin et al., 2018, Han et al., 2005, Lauterbock et al., 2012). In order to create constant concentration gradient, ammonia concentration on the permeate side should always be lower than the feed side, which can be accomplished by a reaction of ammonia with a strong acid to produce ammonium ions in the permeate side (Lauterbock et al., 2012). Ammonia volatilizes at the feed membrane interface then diffuses across the air-filled pores of the microporous hydrophobic membrane (Rezakazemi et al., 2012). Ammonia then diffuses out to the permeate side where it rapidly reacts with a strong acid and forms nonvolatile ammonium salt leading concentration of ammonia on the permeate side to be zero (Rezakazemi et al., 2012, Sancho et al., 2017). As the membrane allows only volatile species to pass through, ammonia ions on the permeate side will not be able to pass through the membrane and thus a perpetual concentration gradient will be accomplished.

Ammonia separation/recovery efficiency through membrane contactor systems or VPMC, depends on the concentration gradient between the feed and permeate phases. Therefore, driving force should decrease as the ammonia concentration decrease in the feed phase (Ahn et al., 2011, Rezakazemi et al., 2012). However, for alkaline feed solutions (pH >10–11), feed ammonia concentration was reported to have negligible effect on separation/recovery efficiency and the free acid concentration in the permeate side was reported to be the main driving force for ammonia separation/recovery (Ashrafizadeh and Khorasani, 2010, Sancho et al., 2017). Therefore, in order to keep ammonia separation/recovery efficiencies high, permeate acid solutions should be replenished regularly to have high free acid concentrations on the permeate side. Decision of implementing membrane contactors such as VPMC to separate/recover ammonia perhaps depends on initial ammonia concentration and target final ammonia concentration in the waste (Darestani et al., 2017). Considering that permeate acid solution should periodically be replenished in order to maintain high ammonia separation/recovery efficiencies, optimum target final ammonia concentration in the feed solution after membrane separation can be decided through feasibility analysis and a polishing step such as low cost biological systems can be used to remove ammonia from the effluents of membrane contactors (Darestani et al., 2017).

Phytoremediation processes are low cost systems compared to conventional chemical or biological ammonia removal systems as there is no chemical requirement. Aquatic macrophyte systems such as duckweed, water lettuce, etc. have been used to remove pollutants from wastewater through phytoremediation (Caicedo et al., 2000). The use of duckweed to remove ammonia from various wastes has been attracted attention recently (Caicedo et al., 2000, El-Shafai et al., 2007, Mohedano et al., 2012, Xu and Shen, 2011, Zhao et al., 2014, Zhao et al., 2015). Duckweed species are small floating plants belonging to botanical family of Lemnaceae with 40 identified species belonging to Lemna, Spirodella, Wolffia, and Wolffiella genera. (Skillicorn et al., 1993). Lemna minor species belonging to Lemna genera was found to remove high quantities of both nitrogen and phosphorus from wastewater (Harvey and Fox, 1973). The unionized ammonia is lipid soluble and easily penetrates through the plant cell membrane (Caicedo et al., 2000). Lemna minor species can accumulate considerable amount of nutrients as proteins and were reported to contain 28.48% crude protein on dry basis, which allows them to be utilized as poultry and other animal feed (Gupta and Prakash, 2013, Harvey and Fox, 1973).

The objective of this study was to investigate ammonia removal from synthetic ammonia solutions and chicken manure digestate through vapor pressure membrane contactor systems and to use phytoremediation process as a polishing step. Ammonia inhibition in anaerobic digesters treating wastes with high protein content is the biggest concern during operation. This study offers a solution to recover ammonia and to prevent ammonia inhibition in digesters treating chicken manure. In this study, ammonia removal through counter-current flow VPMC system with flat PTFE membrane was assessed and operational conditions were optimized to achieve effective ammonia removal. Phytoremediation was used as a polishing step to remove remaining ammonia in the effluent of the VPMC system. To the best of our knowledge this is the first study investigating ammonia removal through combined VPMC and phytoremediation systems.