A review on alternative technologies to manage manure: Cost effective and environmentally beneficial

Ermias Belete and Asrat Ayza

Department of Animal and Range Science, College of Agriculture, Wolaita Sodo University, P. Box: 138, Ethiopia
ermiasbelete30@yahoo.com

Abstract

Livestock production continues to be a very important component of Ethiopian agriculture and environmental issues and play an increasingly important role in the continuous and sustainable development of coming industry. Mostly, dairy farms in the urban parts of Ethiopia continue to grow most of their feed and recycle the manure nutrients on the farm. However, to remain economically viable, many dairy farms are increasing herd size and importing more feed nutrients onto the farm. Despite of all the development and extension efforts in that area, manure management will likely continue to be the top environmental issue facing dairy and beef cattle production in Ethiopia.  Proper integrated manure management is, however not a common practice in most livestock systems leading to loss of nutrients, environmental degradation, human health risks and emissions of greenhouse gases.

Therefore, awareness raising, increasing the knowledge of farmers, extension workers and policy officers; and improving the enabling environment are keys to improving integrated manure management. This review on alternative technologies to manure management practices provides manure as a resource and aware its importance on-farm manure management practices. Further research, development and extension work in livestock manure management must focus on strategies, systems and techniques that allow for maximizing the benefits of manure uses while minimizing its impact of the environment.

Key words: alternative, co-firing, cost effective, integrated, manure, policies, technologies

Introduction

Livestock is one of the fastest growing subsectors of agriculture; a doubling of demand for animal  food source is expected for developing countries and a 70% increase for the world as a whole.  While the livestock sector makes an important contribution to global food supply and economic development, it also uses significant amounts of natural resources and impacts on the environment (Alexandratos and Bruinsma 2012) since environmentally friendly development has become concern to the entire world now (Osak et al 2015).

Livestock producers constantly face the challenge of managing manure especially in urban parts of the country because of the nonexistence of either appropriate place to dispose or appropriate technology to re-utilize animal dung (Asrat et al 2014) and meeting environmental regulations. Animal manures are an important source of organic matter and plant nutrients. The knowledge of manure composition is an important part of good management, either when importing manure onto the farm or transferring nutrients around the farm (Bittman et al 2005).

Manure combustion technology has received academic and commercial interest in the past and it is a proven technology for manure handling (Abelha et al 2003; Font-Palma 2012;  Khan 2009; Lynch et al 2013; Zhu and Lee 2005). Combustion technology was chosen as the alternative for the poultry manure management, first of all, because it has a very good potential to reduce some of the environmental effects that the current management practices are causing, most of all nutrient leaching. In particularly, fluidized bed combustion (FBC) technology was chosen because it can avoid some of the operational challenges that are often related to manure combustion (Khan et al 2009).

Alternative technologies or uses for manure are a compilation of alternative uses for manure from animal feeding operations (AFOs). As this result, reviewing optional manure management serves as a reference concerning the use of alternative technologies to manage manure and utilization of these technologies may assist operators in meeting National Pollutant Discharge Elimination System (NPDES) permit requirements.  

Manure problem and status of technologies

 The idea of looking at manure as a resource, but not waste, has been central to much of the more recent thinking on the whole subject of good farm management and international experience suggests that the development of biogas systems is important for farm waste management. Ethiopia has an abundant of livestock waste resources, but its livestock production management is very inefficient, particularly in the smallholder rural production system? Surpluses of manure also occur in Ethiopia in the urban parts; due to dairy and beef cattle production and this is a major challenge. In the table 1 below, we see current manure situation and its coming technologies.

Alternative technology processes or uses of manure

 Methanol as an energy source

Manure processing is presently a subject that enjoys considerable attention in the world due to the ongoing revision on best available techniques for intensive rearing of livestock, due to the current efforts employed to implement policies and legislation (Foged et al 2011). Methanol is the simplest alcohol, typically made from natural gas. Wood, municipal solid waste and sewage and therefore manure can also be used to produce methanol, which is called biomethanol when made from these sources (American Methanol Institute 2000).  Methanol production is carried out in two steps. First, the feedstock is converted into a synthesis gas stream consisting of carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and hydrogen. In the second step, methanol is manufactured from the synthesis gas. The synthesis of methanol is very exothermic (produces heat), and most plants are designed to use this extra energy to generate electricity needed in the process (American Methanol Institute 2000).

Livestock feed additive

Litter is an excellent source of protein, energy, and minerals, especially for brood cows and stocker cattle. Ruminants have the unique ability to digest forages, other fibrous materials, and inorganic nitrogen such as urea. Because of this ability, by-products of agriculture and the food processing industry can serve as low-cost, alternative feed sources for these animals and one such by-product is broiler litter (Davis 1999). Broiler litter is an economical and safe source of protein, minerals, and energy for ruminant animals when it is processed by an acceptable method. Acceptable methods of processing litter for cattle feed include deep stacking, ensiling, dehydrating, and extrusion-pelleting. Deep stacking is the most common method because it is considered the most economical and the most practical (Carter and Poore, 1996). When litter is stored in deep stacks, heat is produced. Increasing the use of broiler litter as cattle feed increases the distribution of these nutrients from poultry producers to beef producers and increases the profitability of beef production (Carter and Poore 1996). The use of broiler litter to be dried;  as cattle feed is an environmentally responsible use of a by-product and provides an incentive for proper management of this by-product by both poultry and cattle producers (Davis 1999).

Value added products

Composting is the aerobic decomposition of manure or other organic materials in high temperatures known as the thermophilic temperature range (40–65 ⁰C or 104–149 ⁰F) (UNL 1998). During this process, waste and organic matter are allowed to decay in a pile. Compost can be an excellent source of nitrogen, organic matter, and other types of nutrients. The basic factors that influence the rate and efficiency of composting are temperature, water content, carbon to nitrogen (C:N) ratio, aeration rate, and the physical structure of organic materials (particle size). Aeration is important for maintaining composting. As a result, compost is typically piled into 5 to 8 foot-tall wind rows that are turned at 1-60 days intervals (Purdue 1998).  In addition to value added products composting, pelletizing, also known as extrusion, converts fresh manure to a dry, pathogen-free, easy to handle, finished product that can be used as a fertilizer, soil amendment, feed additive, or energy fuel. The manure is compacted at high temperatures and pressures, and then compressed in a die to form pellets. (AgriRecycle 2000).

Conversion as an energy source

Anaerobic digestion is the decomposition of manure in an oxygen-free (anaerobic) environment. Two types of systems are available for on-the-farm anaerobic digestion, anaerobic digesters, and anaerobic lagoons. Anaerobic digesters work in much the same way as an animal’s digestive tract; micro organism’s breakdown or digest the manure (Rabobank Food  and Agri 2013). One of the last phases of digestion is the conversion of the manure into biogas by methane forming bacteria. Biogas is a combination of methane, carbon dioxide, nitrogen, hydrogen, carbon monoxide, oxygen, and hydrogen sulfide (Raven 2004, 2005, 2007).  Between 55 and 70 percent of the biogas is methane, while the remainder consists mostly of carbon dioxide. Usually, the nitrogen, hydrogen, carbon monoxide, oxygen, and hydrogen sulfide are found in trace amounts. Methane in biogas is similar to natural gas, and after scrubbing it can be used to fuel internal combustion engines that run generators and produce electricity (OSU 2000). Green gas . Biogas has received a great deal of attention in academic literature (Geels and Raven 2006, 2007; Markard et al 2009; Negro et al 2007; Negro and Hekkert 2008; Raven and Geels 2010;  and Verbong et al 2001). Although agricultural biogas production capacity has increased tenfold during the 2000-2012 period (Rabobank Food and Agri 2013), “niche development in the Netherlands has shown clear ups and downs, in a non-linear pattern” (Geels and Raven 2006), and is generally considered to have had little success (Geels and Raven 2006; Hofman 2005; Negro et al 2007).

Conversion to co-firing

Co-firing is the simultaneous combustion of a supplementary fuel, such as manure, with a base fuel, such as wood or coal. Co-firing proves to be one of the most promising near-term methods of increasing the use of manure in electricity generation (EREN/DOE 2000). Several types of boiler technologies have been practiced, tested, or evaluated for co-firing, including wall-fired and tangentially designed pulverized coal (PC) boilers, coal-fired cyclone boilers, fluidized-bed boilers, and spreader stokers (EREN/DOE 2000). Stoker-grate firing systems with animal manure have recently and successfully entered the commercial market, typically using a mixture of wood shavings, straw, or both, rather than coal with poultry litter (DOE 2000).

Gasification

Gasification is a process that uses heat to convert animal manure, usually poultry litter, into a clean fuel gas form. The two-step, endothermic (heat absorbing) process converts animal waste into a gas of low- or medium-British thermal units (Btu). This form gives the animal manure tremendous flexibility in the way it can be used to produce power. The first step in the process, called pyrolysis, vaporizes the volatile components of the fuel at temperatures under 600 °C by a set of complex reactions. Fixed carbon, called “char,” and ashes are the unvaporized by-products of pyrolysis. Char is gasified through reactions with oxygen, steam, and hydrogen in the second step. A portion of the unburned char is then combusted to release the heat required for the endothermic gasification reactions. There are several different gasification processes available including; fixed-bed gasifiers, fluidized-bed gasifiers, and low-pressure gasifiers (EREN/DOE 2000).

Land application

 Land application is a critical process in manure management. Pathogens from animal waste can threaten humans who are exposed to runoff, have direct contact with manure, or consume food or water contaminated with infectious manure. Therefore, application rate and seasonal conditions are important factors that contribute to the transfer of pathogens from lands where manure has recently been applied to nearby surface water. However, there is a higher risk of pathogen transfer to the food chain when fresh manure is land-applied than when stored manure is land-applied because there is no storage or treatment period to decrease pathogen numbers (Nicholson et al 2005).

Algae Production

 Biotechnical Europe Limited and Plant Research International of Canada have developed Photosynthetic Purification Technologies (PPT), a biological technology that grows algae and photosynthetic bacteria from the nutrients in animal waste. Photosynthetic Purification Technologies produces a crop of microalgae and other photosynthetic organisms that act as a fertilizer and accelerate the natural growing process and the product can be sold at a profit, while simultaneously providing odor control and producing a clean liquid effluent. Photosynthetic Purification Technologies provides a variety of applications, including a high-protein animal feed supplement (ACFA 2000a).

Aquaculture

 Manure has been used for aquaculture in the Far East for centuries. Studies indicate that while high-protein feed results in higher maximum yields per unit area when compared with manure, high-protein feed costs more. Incorporating manure into high-protein feed results in reduced growth, without reducing feed cost per unit area. Therefore, the best results are obtained from frequent applications of manure alone (ARS 1998).

Bedding or Litter

Livestock waste can be processed into a solid that can be used as bedding or litter. Litter is not alone a mixture of manure, feathers, spilled food, and bedding material be used as bedding materials. Farmers use litter as an inexpensive fertilizer for cropland because the manure contains nitrogen and phosphorus, the two important fertilizer ingredients. A solid liquid separation system is used to separate the solids in the waste from the liquids (Purdue Research Foundation 1996b).

Manure treatment technologies

Physical Treatment

It is sometimes desirable to separate the solid and liquid portions of livestock manure. Solid separation converts the waste into a product that can be sold off the farm, given that a market has been developed (Huebner 1999). Solid separation may be desired to reuse manure solids for bedding or refeeding, to improve the treatment efficiency of vegetative infiltration areas and leach fields, to use the liquids for flushing and to reduce the volume of waste to be hauled. Centrifuges increase the effect of gravity by spinning the manure at high speeds. Centrifuges are small and can produce a substance consisting of 15 to 40 percent solids (OSU 2000).

Chemical Treatment

Manure can be chemically treated to improve solids removal, kill microorganisms, eliminate odors, and limit the spread of disease. Addition of coagulating agents such as ferric chloride, alum, lime, and organic polymers can greatly improve the dewatering characteristics of manure. Coagulants bring manure solids together thus they will settle more quickly. Bringing the small particles together also improves the removal of solids by filtration. Care should be taken when handling coagulants because some are corrosive and while others are extremely slippery if spilled (OSU 2000). Manure can also be treated chemically by raising the pH to about pH 12 for 30 minutes. This treatment kills most of the microorganisms living in the manure, which eliminates odors and the spread of disease. Lime is typically added to raise the pH of livestock manure. A limitation of using lime is that ammonia is immediately lost from the manure. As a result, lime should never be added to manure that is located in a poorly ventilated or confined location (OSU 2000).

Biological Treatment

Biological treatment uses naturally occurring microorganisms in manure to change the properties of waste. Examples of biological treatment include biodrying, anaerobic digestion and anaerobic lagoons, and aerobic lagoons. Biodrying of manure is accomplished by recycling dry compost. It has been proposed that the heat generated in the aerobic decomposition could be used to dry the manure/compost mix with forced air (Hannawald 1999). Anaerobic digestion, which is the decomposition of manure in an oxygen-free (anaerobic) environment, is used in on-the-farm anaerobic digesters and anaerobic lagoons. Digesters breakdown the manure into biogas: that can be collected and used for fuel or energy. Anaerobic lagoons can be covered to collect gas. Un-covered anaerobic lagoons are usually 100–times larger than anaerobic digesters (OSU 2000).

Future manure management challenges

In the past, manure management systems were often considered as externalities to animal production systems, which allowed for the development of highly effective and efficient production systems for livestock (Nowak 2000). Now that manure management systems, with a special emphasis on environmental protection, are increasingly being incorporated into animal production systems. To use an agro-ecological approach that integrates biophysical, technological and human considerations across space and time to optimize livestock production systems in which manure management systems are integrated. Such an approach will require that the cost of environmental protection be included into the prices of animal products for the consumers and this will become an unavoidable challenge in the near future (Miner and Moore 2000).

Conclusions

• In some areas of intensive livestock production, complete abatement of pollution from manure cannot be achieved by improved farming practices alone. A manure treatment technology may have to be part of the solution in order to export nutrients from intensively farmed regions. Future research, development and extension work in livestock manure management must focus on strategies, systems and techniques that allow for maximizing the benefits of manure uses while minimizing the impacts of those uses on the natural resources and ecosystems. The integration of agronomic, economic, environmental, societal and technical considerations by means of a systems approach constitutes one of the best methods for achieving that goal.

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Received 18 June 2015; Accepted 31 August 2015; Published 1 October 2015

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