Treating anaerobic effluents using forward osmosis for combined water purification and biogas production

Highlights

• Biomimetic FO membranes extract water from anaerobic digestion effluents.

• The membranes achieve high ammonia and orthophosphate rejection.

• No trade-off between methane yield and water flux and fouling propensity.

• Organic fouling and biological fouling occurred on the active layer of the membrane.

Abstract

Forward osmosis (FO) can be used to reclaim nutrients and high-quality water from wastewater streams. This could potentially contribute towards relieving global water scarcity. Here we investigated the feasibility of extracting water from four real and four synthetic anaerobically digested effluents, using FO membranes. The goal of this study was to 1) evaluate FO membrane performance in terms of water flux and nutrient rejection 2) examine the methane yield that can be achieved and 3) analyse FO membrane fouling. Out of the four tested real anaerobically digested effluents, swine manure and potato starch wastewater achieved the highest combined average FO water flux (>3 liter per square meter per hour (LMH) with 0.66 M MgCl₂ as initial draw solution concentration) and methane yield (>300 mL CH₄ per gram of organic waste expressed as volatile solids (VS)).

Rejection of total ammonia nitrogen (TAN), total Kjeldahl nitrogen (TKN) and total phosphorous (TP) was high (up to 96.95%, 95.87% and 99.83%, respectively), resulting in low nutrient concentrations in the recovered water. Membrane autopsy revealed presence of organic and biological fouling on the FO membrane. However, no direct correlation between feed properties and methane yield and fouling potential was found, indicating that there is no inherent trade-off between high water flux and high methane production.

Introduction

Access to safe and clean drinking water as well as sustainable energy is a basic human necessity, contributing to human health, poverty reduction and environmental sustainability (Luo et al., 2016; WWAP, 2015).

Despite intense efforts in recent years to increase water supply, sanitation and hygiene for people in water-stressed areas, 663 million people still remain without access to drinking water sources and the population growth outpaces the progress made (WWAP, 2015). Water scarcity is not limited to developing and third world countries, but affects industrialized nations as well. This makes it one of the major challenges of this century and raises the need to develop new sources of water (Shannon et al., 2008).

In addition, the quest to provide sustainable energy sources to satisfy a rapidly increasing global energy consumption while alleviating climate change has yet to be solved (McGinnis and Elimelech, 2008).

One possible solution is the combined reclamation of water and energy from municipal or industrial wastewater sources (Shannon et al., 2008). In recent years, the perception of wastewater has changed. It is no longer considered as waste, but a resource of nutrients (N, P and K), water and energy in form of biogas (Ansari et al., 2017; Lutchmiah et al., 2011).

Biogas can be produced via anaerobic digestion (AD), which converts complex organic matter mainly to methane and carbon dioxide. AD is widely used for the treatment of wastewater because it generates less sludge than conventional aerobic processes and is also more cost-efficient, since aeration is not required and energy can be partly recovered by utilizing the produced biogas (Ansari et al., 2017). In recent years anaerobic membrane bioreactors (AnMBR) that combine biogas production with low-energy wastewater treatment using porous microfiltration (MF) or ultrafiltration membranes (UF) have raised increasing interest. The advantages of using AnMBR systems include improved effluent quality, lower sludge production and improved biogas yields by increasing the retention time of anaerobic microorganisms in the bioreactor (Gu et al., 2015; Wang et al., 2017).

However, conventional AnMBRs face some challenges that are rooted in their reliance on pressure-driven membrane processes and porous membranes (Stuckey, 2012). Less-readily biodegradable soluble organics, dissolved solids (Lay et al., 2010) and trace organic pollutants, such as pharmaceuticals and endocrine disrupting compounds (EDC) (Clara et al., 2005) are washed out through the pores of MF and UF membranes, lowering the effluent quality and negatively affecting the biogas yield (Gu et al., 2015). Further on, fouling results in rapid flux decline and reduces the overall performance (X. Wang et al., 2016). These problems could potentially be solved by replacing MF/UF membranes with tight forward osmosis (FO) membranes.

In FO, a draw solution (DS) is used to induce a net flow of water through a semipermeable membrane into the DS from a feed solution (FS). The flow is driven by the transmembrane osmotic pressure gradient Δπ between the DS and FS and will occur as long as π_DS > π_FS. The π_DS arises from the DS osmolyte where seawater, by-products from industrial processes, and inorganic salts (e.g. NaCl, MgCl₂) all have been evaluated in previous studies. This emerging membrane technology can be used to extract water from wastewater streams while efficiently retaining organic matter and microorganisms (York et al., 1999). FO membrane systems are able to treat complex wastewater streams of varying composition (Lutchmiah et al., 2014), such as landfill leachate (York et al., 1999), municipal wastewater (Hey et al., 2017; Z. Wang et al., 2016), or wastewater from oil and gas separations (Hey et al., 2017). The diluted DS from FO can be re-concentrated by reverse osmosis (RO) (Holloway et al., 2007) or membrane distillation (MD) (Liu et al., 2016), while simultaneously producing high quality water.

Taking all of these considerations together, the integration of FO membranes into an osmotic anaerobic membrane bioreactor (FO-AnMBR) can be seen as a promising technology for wastewater treatment, water reclamation and simultaneous biogas production (Chen et al., 2014; Gu et al., 2015; Li et al., 2017; Tang and Ng, 2014). However, this concept has to our knowledge so far not been tested for high-strength wastewater sources, such as agricultural wastewater and cattle manure.

In this study the membrane performance of a novel biomimetic FO flat sheet membrane is evaluated for the treatment of AD effluents. Eight types of effluents were selected: potato starch wastewater, swine manure and two types of cattle manure (thermophilic and mesophilic), as well as four effluents based on basal anaerobic medium (BA): synthetic sugars, synthetic lipids, synthetic proteins and synthetic mixture. The synthetic effluents were chosen in addition to the real effluents due to their known composition. This should help to find possible correlations between the effluent composition, biogas potential and fouling propensity.

The objective of the present study is to answer the following questions:

• What is the FO membrane performance of the selected AD effluents, with regards to water flux and nutrient rejection?

• What is the methane yield achieved by these wastewaters during AD? This aspect is especially important with respects to reduction of operational expenditure.

• What is the extent and the nature of the fouling and how does the composition of the AD effluents affect the membrane fouling?

Taken together, the results from this study can be used towards the development of an integrated FO-AnMBR-MD/RO process, and will help to improve understanding regarding which types of wastewater can be treated successfully, providing a compromise between high biogas production, good FO-based water extraction and low fouling potential. The scope of the article is depicted in Fig. 1.