Effect of sawdust addition on composting of separated raw and anaerobically digested pig manure

Abstract

Manures need the addition of carbon-rich bulking agents to conserve N during composting, which increases the cost of the composting process. The recommended proportion of manure/sawdust, based on a carbon (C):nitrogen (N) ratio, is approximately 3:2. Two composting experiments were conducted to determine the impact of varying the proportion of sawdust to either separated raw, or separated anaerobically digested pig manures. To determine stability and maturity of the final compost, oxygen uptake rate (OUR) and germination index (GI) tests were conducted. For both experiments, three treatments were employed: manure-only (Treatment A), manure/sawdust mixed 4:1, fresh weight (Treatment B), and manure/sawdust mixed 3:2, fresh weight (Treatment C). The mixtures were composted in tumblers for 56 days with regular turning. The composting material was tested over the study duration for temperature, pH, water content, organic matter, C:N ratio and bulk density. For both Treatments B and C, the GI indicated low levels of phytotoxicity, and OUR values were lower than the recommended Irish threshold of 13 mmol O₂ kg OM⁻¹ h⁻¹, indicating that a high quality compost was produced. The proportion of sawdust to separated manure used can be reduced to make a cost saving, while still producing a stable end-product: 60% less sawdust is required to compost at a manure-to-sawdust ratio of 4:1 compared to the previously recommended ratio of 3:2.

Introduction

Thirty percent of sows in the European Union (EU) are located in a major pig production basin which stretches from Denmark, through north western Germany and the Netherlands to Vlaams Gewest in northern Belgium (Marquer, 2010). Other important regions include Cataluña and Murcia in Spain, Lombardia in Italy, and Brittany in France. In the Republic of Ireland, 38% of the national sow herd is concentrated in counties Cork and Cavan (Boyle, 2010). Pig manure in these concentrated pig farming areas must be transported to less animal dense areas for landspreading, thereby increasing the cost of manure handling. As a result of the EU Nitrates Directive (91/676/EEC; EEC, 1991), the amount of livestock manure which can be applied to land has been limited to 170 kg of nitrogen (N) per hectare per yr. In Ireland, the land available for landspreading will further be restricted, starting in 2013, and culminating in 2017, when landspreading of pig manure can no longer exceed the crop's phosphorus (P) requirements for growth (S.I. 610 of 2010). The implication of this will be that an additional ∼50% land area will be required for manure application than is the case in 2012. The resulting increase in manure transport costs for farmers, along with the potential of surface and groundwater pollution from the landspreading of manure, has resulted in the need to examine practical and economical on-farm solutions for swine wastewater treatment. Recently, anaerobic digestion (AD) has become topical as a means of producing energy from farmyard by-products, including pig manure. However, AD does little to reduce the nutrient content of pig manure, which still needs to be recycled in the same way as undigested manure. One option may be to compost pig manures to produce a high quality, marketable product.

Composting of manure requires separation of the liquid manure to produce a solid and liquid fraction. The solid fraction concentrates the P and can be composted. Composting has the potential to stabilise the organic N fraction of manure and increase its fertiliser value, while, at the same time, reducing its volume and odour, making it cheaper and easier to transport (Bernal et al., 2009). The stabilisation of the OM in the composting materials determines the effectiveness of the composting process. For stabilisation to occur, key factors, such as temperature, aeration, water content (WC), pH and structure must be at an optimum level both initially and throughout the composting process. The C:N ratio is one of the most important factors influencing the quality of compost produced (Zhu, 2007). Sweeten and Auvermann (2008) recommend a C:N ratio of 20–30, while Rynk (1992) recommended 25–30. Since the C:N ratio of separated pig manure is reported to be 11.3 (Huang et al., 2006), the addition of C-rich bulking agents is required to provide optimum C:N conditions. Previous studies have looked at the effect of C:N ratio on composting of manures; however, composting of manure after AD has not been investigated.

Studies have found that the solid fraction from mechanically-separated pig manure was too wet to be composted alone and, therefore, required the use of low-moisture bulking agents (Georgacakis et al., 1996; Nolan et al., 2011). Bulking agents generally have low water and high C contents (Bernal et al., 2009) and, when added to manure before composting, act to increase the C:N ratio, decrease the WC, and improve the structure, porosity and free air space (FAS) of the composting mix. Nolan et al. (2011) investigated the composting of separated pig manure using chopped straw, sawdust, greenwaste and woodchip as bulking agents. Sawdust appeared to be the bulking agent which resulted in the most stable compost. However, the addition of sawdust adds an extra cost to the composting process (Nolan et al., 2012).

There are many different methods used to test compost quality including: germination index (GI) (Tiquia, 2005; Zhu, 2007), oxygen uptake rate (OUR) or CO₂ production rate (Wang et al., 2004), water soluble organic C: total organic N ratio (Hue and Liu, 1995; Bernal et al., 1998) and degree of OM humification (Hue and Liu, 1995). Industry-led quality standards for biodegradable material-derived compost are currently being developed for Ireland (Prasad and Foster, 2006). As part of these standards, an OUR test has been recommended for measuring compost stability. As manure-based compost will have to adhere to these new standards, it is imperative that farmers are provided with the necessary information to enable compliance. There are currently no European standards for compost and growing media (Baumgarten, 2011). However, this may not be the case in the future as the European Peat Media Association has called for standards to be developed. These standards would likely be based on CEN test methods, including EN 106086-2, Determination of plant response (cress seed germination test) and EN 10087-1, Determination of the aerobic biological activity (OUR test) (Baumgarten, 2011). The aim of this study was to investigate the effect of adding different quantities of sawdust as a bulking agent to separated raw and anaerobically digested pig manures on the physico-chemical properties, maturity and stability of the compost produced. Compost maturity was measured using a GI test, while stability was measured using an OUR test.