Nutrient recovery from sludge using filtration systems

Lovitt, R., Zacharof, M.P., Gerardo, M.L., Swansea University


Abstract The fluids remaining after the digestion are a significant and growing problem as they contain large amounts of ammonia and phosphate that have high environmental impact. However, these nutrients are of considerable value replacing expensive manufactured fertiliser which typically have large carbon footprints and whose cost is linked directly to that of natural gas.

Typical disposal of these fluids is by land spreading, but the capacity for this is limited as it threatens to contaminate ground and surface water causing eutrophication. Alternative practices of disposal are problematic as they need to be either integrated with AD sites or the materials be transported from digester site to a point of use. Transport soon becomes prohibitively expensive as the solutions are too dilute and bulky. Concentrating and reformulation of the nutrients as fertiliser or growth nutrient can add considerable value to these fluids so allowing transportation and distribution. As a first step in this process the sludge fluids must therefore be separated and concentrated.

The filtration of sludge obtained from digesters offers potentially low cost techniques by which solid materials can be separated from the fluids and thus represent the first step towards recovery concentration and reformulation of these fluids as fertilisers and nutrient. Conventional filtration is used to remove large particulates > 0.5 mm but there several process advantages in post processing if the clear fluids are produced.

This report discusses experiments carried out on several sources of sludge using a ceramic crossflow microfiltration. The aim of these studies were therefore to ascertain the suitability of cross flow systems for sludge filtration and develop and optimize techniques by which these systems could be used to obtain particulate free fluids conducive to further processing. We report the use of a pilot-scale filtration system that has processed 100L batches of agricultural and municipal sludge for nutrient extraction. We have investigated several strategies for nutrient recovery that allow the production balanced nutrient streams. We have been able to use process models that are able to predict processes performance and optimisation based upon the membrane flux using a membrane resistance model. We also discuss further process alternatives to produced concentrates as liquids and solids.

Keywords: membrane, cross flow, microfiltration, anaerobic digester sludge, Phosphate, Ammonia, recovery,

Introduction: Millions of tonnes of sewage are produced every day in Wales and England, and there has been much drive to reduce its environmental impact (Environment Agency, UK). Its treatment,handling and reuse can be challenging and expensive due to the complex and dynamic nature of the wastewater. In addition, the disposal of sewage sludge must take into consideration economic, social and environmental aspects (Tchobanoglous and Burton, 1991). The composition of sewage sludges is complex, however potentially, these can be a valuable source of energy and materials such as nutrients and metals. The production of methane has been widely explored through AD of manure waste from dairy farms and municipal waste sludge (Borja, 2011, Borole et al., 2006, Tezel et al., 2011). The AD of sewage sludge also benefits from a conversion of organic nitrogen into plant available ammonia nitrogen, however phosphorous is usually bound to organic matter or tied up in insoluble materials. Phosphorous therefore, may not be easily available and require processing to be released (Cantrell et al., 2008, Honeycutt, 2004). Seemingly, nutrients from AD sludge can be a valuable resource in agricultural applications, production of microalgae (Cho et al., 2011, Christenson and Sims, 2011, Park et al., 2011) and in hydroponic systems (Mavrogianopoulos et al., 2002, Yang et al., 2008).

The use of sludges as a source of nutrients is not simple due to their complex nature, high solids content, organically bound material and ultimately its potentially hazardous properties. Typically, in farming, the AD sludge undergoes a simple solid-liquid separation process which allows the application of the different fractions as fertilisers in either farm or grassland. This continuous and inadequate disposal of AD sludge can easily result in the eutrophication of the ecosystem due to loses of nutrients into the environment (Wilcock, 2004, Neal and Heathwaite, 2005). Due to the noticeable soil acidification and ecosystem degradation, policies for such disposal of AD sludge are therefore becoming more stringent (Giola et al., 2012, Mantovi et al., 2006).

Membrane separation processes related to water treatment have been long established, however, only recently researchers have reported on the use of membrane filtration for the treatment of wastewater. Tay et al. (1995) demonstrated the feasibility of membrane filtration to decrease the level of contaminants in wastewater from the beverage industry by more than 90%. Other researchers have also achieved similar results in which chemical oxygen demand and total dissolved solids were reduced by more than 90% (Galambos et al., 2004). The are many advantages of membrane filtration and these include lower operating and maintenance costs, physical separations free of chemical additives, potential particle and pathogens free permeates (Chen et al., 2006). Typically, cross-flow filtration is used to avoid the problems encountered with dead-end filtration such as cake fouling and high energy consumption. (Wang et al., 2006).

The recovery of nutrients from a multi-solute system in a particle and microbe free solution is not without challenges. MF membranes range from 0.1 µm to 1 µm and allow to obtain particle and microbe free solutions in which smaller particles such as aqueous salts, proteins, metal ions, and other inorganic and organic particles can be collected as solutions (Wang et al., 2006). Apart from separating certain solutes from a solution, membrane filtration can also be used to concentrate and purify the final product. DF is a membrane assisted process that allows the recovery, concentration and purification of materials from a multi-solute system (Fikar et al., 2010). Diafiltration (DF) consists of the addition of wash-water to the multi-solute system performed in batch mode, continuous mode or counter current mode. The application of a DF mode is case-specific and thus every multi-solute system will present itself as an unique challenge (Lipnizki et al., 2002).

Herein we investigate the recovery nutrients, using filtration processes, from two different sources: manure sludge from a dairy farm and sludge waste from a trout aquaculture farm. This work reports on the filtration characteristics for both types of sludges, nutrient recovery strategies using acidification and DF, and a model that relates filterability, costs and nutrient recovery is also presented.

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