Closing the Nutrient Cycle by Using Bio-Based Amendments as Substitutes for Fossil-Based Fertilizers

TABLE OF CONTENTS

ACKNOWLEDGEMENT

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

DEDICATION

ABSTRACT

CHAPTER ONE
1.1 BACKGROUND OF STUDY
1.2 PROBLEM STATEMENT
1.3 SUBJECT JUSTIFICATION

CHAPTER TWO
2.1 INTRODUCTION
2.3 DIGESTATE PRODUCTION AND THEIR NUTRIENT CONTENT
2.4 DIGESTATE SEPARATION
2.5 NUTRIENT ELEMENT NITROGEN RECOVERY
2.5.1 AMMONIA STRIPPING
2.5.2 AIR SCRUBBER

CHAPTER THREE
3.1 SOIL SAMPLE PREPARATION
3.2 CHEMICAL ANALYSIS
3.2.1 NITRATE
3.2.2 SOIL pH
3.3 ELECTRICAL CONDUCTIVITY (EC)
3.4 TOTAL NITROGEN
3.5 MAJOR NUTRIENTS
3.6 AVAILABLE MAJOR NUTRIENTS
3.6.1 SOIL ANALYSIS
3.6.2 PLANT ANALYSIS
3.7 STATISTICAL ANALYSIS

CHAPTER FOUR
4.1 SOIL QUALITY
4.2 SOIL NUTRIENT STATUS
4.3 PLANT NUTRIENT CONTENT

CHAPTER FIVE
5.1 IMPACT OF BIO-BASED AMENDMENT ON SOIL FERTILITY AND SOIL QUALITY
5.1.1 SOIL QUALITY
5.1.2 SOIL FERTILITY
5.2 IMPACT OF BIO-BASED AMENDMENT IMPACT PLANT NUTRIENT CONTENT
5.3 CONCLUSION

REFERENCES

APPENDIX

ACKNOWLEDGEMENT

This work has been made possible thanks to the valuable contribution of many persons. Words of gratitude to all those who lend me a helping hand in one way or the other. Much thanks goes to my promoter and supervisor, Prof. Filip Tack, and Céline Vaneeckhaute who both took patience in going through my work. I thank them for their words of encouragement and advice that led to the accomplishment of this work. Special thanks to the laboratory technicians of the Laboratory of Analytical and Applied Ecochemistry; Joachim Neri, Katty Sabo, David Lybaert and Ria Van Hulle. Further gratitude goes to all professors in the Physical Land Resource Programme for their immense contribution to this work and have collectively been very instrumental during my studies.

My utmost thanks goes to my father, Mr. Ndze Wongbi Tobias who made a lot of sacrifices both financially and morally and has been willing to provide for me at any moment all the possible materials needed for success. I also wish to extend gratitude to the rest of my family and well wishers. To my classmates who have in one way or another helped me go through difficult moments; I say thank you. Above all the greatest thanks go to the Almighty God for divine words in the scripture that gave me courage and much hope.

LIST OF TABLES

Table 1. Products from liquid manure, digestate and scrubber water used for the field trial ..

Table 2. Doses of products per hectare in the different treatments

Table 3. Dose of effective nitrogen in the products per hectare in the different treatments

Table 4. Dose of total nitrogen in products per hectare in the different treatments

Table 5. Dose of total phosphate in products per hectare in the different treatments

Table 6. Total doses of products per field

LIST OF FIGURES

Figure 1.Schematic representation of a stripping system

Figure 2. Schematic representation of an air scrubber. Source (Melse et al., 2009)

Figure 3. The average NO3-residues per scenario in the soil (0-90 cm)

Figure 4. Soil electrical conductivity per scenario before and after maize was harvested

Figure 5. SAR levels for different scenarios before and after maize harvest

Figure 6. Total soil nitrogen content before and after maize harvest

Figure 7. Total soil phosphorus content before and after maize harvest

Figure 8. Total available soil phosphate content before and after maize harvest

Figure 9. Total soil calcium content before and after the maize harvest

Figure 10. Soil available calcium content before and after maize harvest

Figure 11. Total soil magnesium content before and after maize harvest

Figure 12. Soil available magnesium content before and after the maize harvest

Figure 13. Total soil potassium content before and after maize harvest

Figure 14. Total soil available content potassium before and after maize harvest

Figure 15. Total sodium content before and after maize harvest

Figure 16. Total soil sulphur content before and after maize harvest

Figure 17. Plant nitrogen build-up before and after maize harvest

Figure 18. Plant phosphate content before and after maize harvest

Figure 19. Plant sodium content before and after maize harvest

DEDICATION

This work is dedicated to God Almighty and my beloved parents Mr. and late Mrs Wongbi and to all my family members for their moral support throughout my stay in Ghent University.

ABSTRACT

In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recycle valuable nutrients that currently end up in waste streams. Nutrient resources are rapidly depleting. Significant amounts of fossil energy are required for the production of synthetic fertilizers, whereas costs for energy and fertilizers are increasing. The goal was to evaluate the impact of bio-amendments instead of synthetic fertilizers. It aims to i) identify the impact of the different scenarios on the soil fertility ii) the influence of bio- amendment substitutes on the soil quality and iii) the impact of the applied amendment on the plant nutrient contents. Eleven scenarios were designed from liquid manure, digestates and scrubber water. Their application resulted in no nitrate pollution according to the Flemish Manure decree in the top 90 cm of the soil. Also there was no negative impact on the soil quality and soil fertility though soil degradation needs to be monitored in the long term. Moreover, the nutrient status of the experimental field was already quite high to begin with. However, the lack of a reference treatment precluded the real evaluation of the potential of the bio-amendments on soil quality and crop yields. Meanwhile, biogas production through anaerobic digestion produces nutrient rich digestates, which could potentially be reused as green fertilizers in agriculture, thereby providing a sustainable substitute for synthetic fertilizers.

CHAPTER ONE INTRODUCTION AND CONTEXT

1.1 BACKGROUND OF STUDY

Manure can be used to improve the nutrient status of agricultural fields. In the Flemish agricultural sector, the manure policy is one of its most important environmental policies (Van der Staeten & Buysse, 2009). Biodegradable waste management, has become one of the main target fields in this progression. Manure can be utilized as a substitute for inorganic fertilizers. Fossil fuel energy is needed for the production of inorganic fertilizers. It is usually expensive. In 2011, 10.2 million tons of nitrogen (N), 2.2 million tons of phosphate (P2O5) and 2.4 million tons of potash (K2O) were applied to 134.4 million hectares of farmland in the European Union per season (EFMA, 2011). By 2020/21, Fertilizers Europe forecasters expect that these fertilizer consumption figures will reach 10.8, 2.6 and 3.2 million tons respectively, applied to 133.7 million hectares (EFMA, 2011). Minerals such as P and K now extracted via mining, are rapidly becoming limited (EFMA, 2000).

Intensive livestock and agriculture is been practised in Belgium and other European countries. The practice of intensive animal agriculture in some areas has resulted in excessive manure production for the available land base (Ippersiel et al., 2012). In line with the cradle- to-cradle approach, the consideration of biodegradable waste as a secondary resource for the production of green fertilizer substitutes for fossil-based mineral fertilizers has become an important course in the path to sustainability (Vaneeckhaute et al., 2011). Livestock manure is an important source of calcium (Ca), magnesium (Mg), potassium (K), sodium (Na) and nitrogen (N) for crop production. It decreases the need for mineral fertilizer use, particularly in intensive livestock production (Alitalo et al., 2012). Intensive livestock and arable crop production contribute substantially to the economy of European countries in terms of employment and export to other countries (Melse & Timmerman, 2009). This is also true for the Belgian economy. However, animal manure production is connected to a number of environmental effects, especially when poorly managed. Nutrient leaching, mainly nitrogen, phosphorus, ammonia (NH3) evaporation and pathogen contamination are huge threats to the immediate environment (Holm-Nielsen et al., 2009).

Mineral fertilizer production of nitrogen requires significant amounts of fossil fuel energy. However, fossil fuel utilization as energy for fertilizer production no longer guarantees a sustainable future for improving soil fertility. Manure provides a logical answer to improve the fertility status of soils. Manure should be considered as a valuable fertilizer source. It is a component of sustainable agriculture, not disposable waste. During anaerobic digestion manure is mostly used. This is a well-known technology that breaks down degradable material in the absence of oxygen. It produces profitable products such as biogas and digestates. It greatly reduces relevant soil, water and air pollution problems (Fricke et al., 2007; Abdullahi et al., 2008; Ahlgren et al., 2010; García-Bernet et al., 2011; Frigon et al., 2012; Wang et al., 2012).

Digestates is a waste by-product produced during this industrial process (Ahlgren et al., 2010). Digestates can also be used as fertilizers. According to Ippersiel et al. (2012) manure which is composed of approximately 90% water, sometimes has to be transported over long distances at great cost. Moreover, farmers are most often reluctant to apply manure on their land because of its low and unbalanced nutrient concentrations. Most farmers care less about the resulting environmental effects instead focussing their attention on how they can increase crop yields. Manure can be treated in order to ensure that it is more acceptable for farmers. Moreover, it can be in conformity with several legislations that have been put in place.

Some of these directives include the current European Union (EU) legislation (Nitrate Directive 91/676/EEC) (Klop et al., 2012); in Belgium the manure policy has been adopted (Van der Staeten & Buysse, 2009) and in Japan recycling of food waste was accelerated by the introduction of the “Act on Promotion of Recycling and Related Activities for Treatment of Cyclical Food Resources” (the food Waste Recycling Law) (Yabu et al., 2011). Local Flemish and international regulations all point to a reduction of the environmental load on livestock farms. These legislations and laws have lead developers to think about creative ways of extracting nutrients from manure as several innovative technologies have been designed. Anaerobic digestion has been reported by several authors as been a way of processing animal manure (Arthurson, 2009; Antonini et al., 2011; Yilmazel & Demirer, 2011; Vaneeckhaute et al., 2013b).

1.2 PROBLEM STATEMENT

Significant amounts of fossil fuel energy are used to produce inorganic fertilizers. Fossil fuels are depleting and fossil fuel energy is becoming increasingly expensive. The inequalities between the demand and supply for fertilizers will subsequently raise their prices. Therefore, inorganic fertilizers are tagged with increasing prices. Local farmers have to spend more in order to purchase the same quantities of fertilizers they apply on their agricultural crops. Applying livestock manure in order to main/improve the soil nutrient status is logical reasoning which has been used by farmers. However, in the Flanders, the manure policy is one of the most important environmental policies in the agricultural sector. Therefore anaerobic digestion technology can be applied.

This residue from anaerobic digestion process does not reduce the nutrient concentration from livestock manure (Klop et al., 2012). Digestates is also considered as manure according to the manure policy. Digestates just like manure can be considered a valuable source of fertilizer, but their nutrient concentrations are poorly balanced. Liquid manure has to be transported over large distances at great cost even though they are rich in nutrients. Nutrients such as nitrogen, and phosphorus supplied from manure and digestates may accumulate in the soil and water leading eutrophication which involves excessive algal growth (Melse & Timmerman, 2009). This can lead to potential adverse effects on biodiversity or human uses of waters.

Phosphorus reserves on the planet are limited (EFMA, 2000). If more manure/digestates are applied on agricultural fields to meet the nitrogen needs of most crops, a substantial build up of phosphorus occurs on the soil (Moller et al., 2007). Gaseous emissions in the form of ammonia can be toxic to aquatic life if surrounding waters are contaminated (Carew, 2010; Cheng et al., 2011; Fu et al., 2011; Değermenci et al., 2012).

Sustainability can be ensured if nutrients are maximally recuperated from manure/digestates in order to produce a bio-based fertilizer. This bio-based fertilizer should reduce the high cost usually associated with the production of inorganic fertilizers. Diverse approaches which take into account all the physic-chemical properties of manure/digestates should ensure that bio-based fertilizers are produced at a lower cost.

1.3 SUBJECT JUSTIFICATION

Sustainability can be referred to as the utilization of present natural resources to meet our needs without compromising the chances of future generations to meet their own needs. Closing the nutrient cycle will mean several substitutes for fertilizers will need to be applied to arable land.

In Belgium and many parts of the world, digestates constitute an important source for nutrients. In the Flanders, digestates cannot or cautiously added to the soil in its unprocessed form. Digestates need to be processed in order to permit their full use on arable land. It is a sustainable alternative to mineral fertilizers (Vaneeckhaute et al., 2012). Currently, digestates and other valuable waste by-products need to be investigated in order to see how these new products could substitute for traditional mineral fertilizers. Once it has been shown they can be used, authorities need to be convinced to recognise these products as fertilizers.

Soil features such as the pH, salinity, nitrate content and the composition of the main nutrients in the soil could be influenced by the application of digestates. The goal of this dissertation is to seek out ways to utilize digestates and other bio-based amendments as substitute for mineral fertilizers. It aims to i) identify the impact of the different scenarios on the soil fertility ii) the influence of bio-amendment substitutes on the soil quality and iii) the impact of the applied amendment on the plant nutrient contents. These ccomparative studies will be conducted to determine the effects of applying bio-based amendments on the soil and plants nutrient contents.

CHAPTER TWO LITERATURE REVIEW

2.1 INTRODUCTION

The maximal recovery of valuable nutrients coming from waste streams is now an important challenge in the transition from a fossil to a green based economy. Nutrient resources are rapidly depleting. Nutrient resources are required to produce mineral fertilizers. The significant costs and amount of fossil energy applied to produce chemical fertilizers is constantly rising. Hence, chemical fertilizers are tagged high prices. It probably suggests that the high demand for fertilizers produced by highly priced fossil fuels only increases the prices the mineral fertilizers produced. It is therefore essential for societies to move from a fossil fuel to a bio-based economy which incorporates a sustainable resource management.

European member states are being encouraged to utilize biodegradable waste to reduce greenhouse gas (GHG) and to improve the efficient use of natural resources (Havukainen et al., 2012). The consideration of biodegradable waste as a secondary resource can be used to produce profitable eco-friendly products. Green fertilizers can be one of these products. They will contribute in the transformation of societies to a more bio-based economy. Moreover, agro-ecosystem services of the arable land may be improved. In the EU-27 countries over the last three growing seasons, on average fertilizers containing 10.5 million tonnes of N, 2.4 million tonnes of phosphate (P2O5) and 2.7 million tonnes of potash (K2O) was applied each season (EFMA, 2012). The EFMA (2003) suggested that about 29 gigajoules (GJ) is used to produce one ton of ammonium (NH4) at optimal conditions under the Haber-Bosch process.

The conversion of manures from stables, crop residues, wastes from food industry, municipal wastes and steadfast energy crops are the main feedstocks for anaerobic digestion (AD) in biogas facilities (Möller & Müller, 2012). AD has been evaluated as one of the most energy efficient and environmentally friendly technologies for the production of energy. In nutrient-rich regions such as The Flanders (Belgium), Barcelona (Spain), Bretagne (France), Netherlands and Denmark, unprocessed digestate cannot be applied directly to arable land (Lemmens E. et al., 2007). According to Lemmens E. et al. (2007), manure processing provides a solution for the excess manure produced in livestock rearing. The Flanders suffer from a strong discrepancy between the amounts of nutrients produced in stables and the amount of manure that can be used on arable land. Therefore concerns about over-fertilization have greatly complicated the development of bio-digestion. Apparently, manure processing is Flanders aims at neutralizing the nutrients in the manure such the conversion of nitrates into nitrogen or improving their value in order to make them suitable for export to areas requiring organic fertilizers.

Anaerobic digestion is an established technology that converts biodegradable waste into cost-effective by-products as well as reducing significant soil, water and air pollution problems (Lei et al., 2007; Ward et al., 2008; Ahlgren et al., 2010; Tambone et al., 2010; Zhang & Jahng, 2010; Guštin & Marinšek-Logar, 2011; Yilmazel & Demirer, 2011). Most researchers agree that anaerobic digestion eliminates mainly carbon (C) and methane (CH4), leaving behind nitrogen and phosphorus almost intact. During anaerobic digestion, about 20 - 90% of the feedstock organic matter is degraded (Möller & Müller, 2012). Organic matter degradation will depend on the feedstock composition. The remaining nutrients in the digestates can be recovered. Yilmazel and Demirer (2011) point to the fact that there is an on- going shift from the removal to the recovery of nutrients as concerns regarding limited natural resources increase. This promotes sustainability.

As already noted digestates are rich in nutrients and anaerobic digestion has been identified as one of the most cost-effective technology to maximally process animal manure. The aim of closing the nutrient cycle by using bio-based amendments as substitutes for fossil- based fertilizers can permit crop producers to reduce their cost on synthetic fertilizers. The application of bio-based substitutes as fertilizing agents which are recycled back to arable land ensures that crops receive the majority of the essential nutrients required for growth. Soil fertility can be preserved and the soil structure and humus balance will be amended (Arthurson, 2009), thus promoting the closure of the natural nutrient cycle. The benefits of balanced fertilization using bio-based amendments in maintaining soil organic levels in soils, is increasingly emphasized by researchers (Tambone et al., 2010). In other words, they resolve most issues already confronted when synthetic fertilizers are produced. Inorganic fertilizers applied to crops on fields is supplementary to the nutrient cycle, resulting in the need for increased production of fertilizers which require significant energy.

This research proposal aims at exploring how nutrients in bio-based amendments can synthetic fertilizers in other to close the nutrient cycle. Balanced fertilization is aimed at employing synthetic nitrogen fertilizers, animal manure, air scrubber water, solid and liquid digestate fractions, urine and liquid manure on several plots in ten (10) scenarios and one reference scenario. Suggestions will be put forward primarily if the are suitable for agricultural production. Key knowledge and issues on the production, fertilizing status and the effect of animal manure, digestates, solid and liquid fractions of digestates, and urine will on the soil and plants will be addressed. Overall, the application of bio-based substitutes as fertilizers within agriculture has not been extensively evaluated as a field experiment as other types of organic waste.

2.3 DIGESTATE PRODUCTION AND THEIR NUTRIENT CONTENT

Anaerobic digestion (AD) has the benefit of producing biogas which is used for energy production. However, residues are also produced. These residues are generally referred to as digestates. According to Arthurson (2009), AD of organic waste has the potential to significantly reduce global warming and climate change. It promotes enhanced cycling of nutrient resources through nutrient-rich digestates. They present an alternative to the energy demanding generation of inorganic fertilizers. The amount of digestate generated is parallel with the increase of anaerobic treatment for biodegradable waste (Alitalo et al., 2012). Biodegradable waste is sometimes referred to as feedstock or substrate. The feedstock can be either a single input such as only animal manure or a mixture of two or more feedstock types. Depending on the feedstock composition, 20-95% of the feedstock organic matter (OM) can be degraded (Möller & Müller, 2012). Anaerobic digestion feedstock contains nutrients.

These nutrients can either be macro- or micronutrients although some heavy metals and persistent organic compounds can be present in varying amounts. The major macro- nutrients will include calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), phosphorus (P) and sulphur (S). Anaerobic digestion removes carbon mostly in the form of carbon dioxide (CO2) and methane (CH4) gas. This contributes in the fight against global warming and climate change as it reduces the amount of carbon dioxide and methane discharged into the atmosphere. This leaves behind a significant amount of nutrients principally nitrogen in the form of ammonia (NH3) in digestates. Other nutrients which can found in digestates are phosphorus and potassium. Several authors testify to the fact that phosphorus and potassium have been recovered mainly through struvite precipitation (Greaves et al., 1999; Wilsenach et al., 2007; Antonini et al., 2011; Mangwandi et al., 2013).

To assure the maximal recovery value of biodegradable waste, the digestate should have a meaningful purpose, and optimal benefits should be derived from its production. Digestates are either directly spread as manures or treated. When treated it can be separated into solid-liquid fractions; it can be dried, diluted or filtrated before been applied onto arable land (Möller & Müller, 2012). However, digestates cannot be applied directly to arable land in the Flanders, Belgium. Another aim of treating feedstock is to minimize transportation costs (Melse & Timmerman, 2009). One or more components such as phosphorus and dry matter can be accumulated into a fraction of the original volume. This small fraction can be transported at relatively low costs to low nutrient regions where it can be applied on arable land as fertilizer.

2.4 DIGESTATE SEPARATION

Digestate treatment is the processing of digestates into exportable end-products or dischargeable water. The possible techniques presently available for digestate treatment essentially comprise of separation and/or dewatering techniques. Hjorth et al. (2010) suggests the use of emulsion or power based polymers for flocculation. This results into solid and liquid fractions. The liquid fraction contains the majority of the nutrients including inorganic nitrogen (Vaneeckhaute et al., 2011).

Digestate is one of the by-products of anaerobic digestion. Digestate can be separated mechanically and non-mechanically. Moller et al. (2007), compares the separation between and rotatory screen separation. The screw press differentially partitions more dry matter, volatile solids, carbon, ash and phosphorus to the solid fraction than the liquid fraction. Nitrogen (N), ammonia (NH3) and potassium end up mostly in the liquid fraction. In order to improve nutrient use efficiency and safety of digestates, appropriate post-anaerobic digestion treatments are necessary in promote characteristics suitable for agricultural use.

The afore-mention points suggest that the liquid may significantly improve the nutrient status of the soil. The nutrient use efficiency of each nutrient may increase when the liquid fraction is used. The solid fraction may on the other hand add much needed phosphorus to the soil and improve the physical qualities of the soil. Qualities such as permeability, and aeration will be greatly improved as well as the soil organic matter will be greatly enhanced. Vaneeckhaute et al. (2011) and Ippersiel et al. (2012) suggest that further steps in treating the liquid fraction of digestate could be ammonia stripping and/or membrane filtration. Membrane filtration examples can be microfiltration, ultrafiltration and/or reverse osmosis. Each step again generates two by-products downstream: concentrate and permeate. The permeate and concentrate have different characteristics concerning macro- and micronutrient composition. Alternatively, the liquid fraction after separation can be treated biologically through nitrification-denitrification processes. However, in regions where nitrogen emissions from arable land are already extreme, the biological method is not desirable environmentally and economically since valuable nitrogen will be converted into nitrogen gas, which is then eliminated from the local agricultural cycle (Vaneeckhaute et al., 2011). The liquid fraction can be used as a liquid fertilizer or undergo ammonia stripping in order to strip and use specifically the nutrients contained.

The solid fraction of digestates can be considered as an organic amendment with high fertilizing effects having less risks on the environment. The continuous reduction of arable lands and the risks of surface and groundwater pollution by surface runoff and leaching can promote anaerobic digestion sites to move towards digestion techniques which produce more solid fraction. This presents the benefit to be more economically handled and transported (Ippersiel et al., 2012). Solid fractions have a significant amounts of nitrogen as total nitrogen is conserved during the anaerobic digestion. This can bring about management issues as ammonia can be stripped from these solid fractions after digestion. Teglia et al. (2010) suggests that indicators used to define compost quality in order to identify relevant parameters that will characterise a direct use of the solid fraction and/or their post anaerobic digestion treatability.

The application of scrubber water can be essential in supplying nutrients to the arable land. They are normally produced after a target nutrient such as ammonia has been collected. The next topic tries to give a detailed summary of the stripping process and how air scrubber water can be obtained.

2.5 NUTRIENT ELEMENT NITROGEN RECOVERY

Diverse approaches based on the physicochemical properties of digestate have been investigated to isolate and concentrate the ammonia contained in the liquid fraction after separation. Ammonia and its compounds cause major environmental problems when discharged in natural waters without treatment. Değermenci et al. (2012), suggests that even small amounts of ammonia have negative effects on aquatic life. A prerequisite for efficient ammonia removal is that the pH of the liquid to be stripped is sufficiently high.

Nutrient element nitrogen recovery technologies include ammonia stripping towers or air stripping, biological denitrification, steam stripping, electrodialysis, selective ion exchange, membrane separation, direct aeration and breakpoint chlorination (Zhang & Jahng, 2010; Resch et al., 2011; Alitalo et al., 2012; Ippersiel et al., 2012). The choice of a particular ammonia removal method may depend on the nature of liquid fraction after separation. Zeng and Xiaomei (2012) provides us with some few drawbacks concerning the technologies in place. Biological denitrification is hindered by low-temperature environments, the absence of carbonaceous compounds in suitable amounts and the presence of toxic compounds. Interfering ions present in the liquid fraction can negatively affect an ion exchange. Breakpoint chlorination is usually too expensive for practical uses unless the amount of ammonia to be removed is small, which is not feasible if profit is to be made by maximally recuperating nutrient element nitrogen.

2.5.1 AMMONIA STRIPPING

Ammonia stripping is one solution to remove ammonia. Ammonia stripping is a simple desorption process used to lower the ammonia content of a liquid phase (usually a waste water stream) in a counter-current tower (USEPA, 2000). Zeng and Xiaomei (2012) considers ammonia stripping as the recovery of nutrient element nitrogen in various forms, including (but not limited to) its gaseous form (ammonia gas), the various ammonium salts, or other nitrogen-containing chemical forms. It is also expected that this technique can be applied to the liquid fraction after separation of digestates. Digestate is rich in nutrient element nitrogen, which partly originates from the degradation of nitrogen rich proteins peptides and amino acids present in the original biodegradable waste. Most liquid fractions have a large amount of ammonium and/or ammonium-containing compounds that readily form ammonium. It is easier to recover nitrogen using ammonium than to convert it nitrate- nitrogen before removing it. Ammonia is a weak base 14 which reacts with water (a weak acid) to produce ammonium hydroxide (NH4OH). Zeng and Xiaomei (2012) suggest that a purification step can be carried out prior to the ammonium stripping process in order to increase the concentration of ammonium in the liquid fraction. This facilitates an easier and a more complete stripping.

At pH 7 (at 20 °C), only 0.4 % of ammonium is in the form of ammonia. It is necessary to raise the pH to achieve efficient ammonia removal. Manure has a high buffering capacity and large amounts of chemicals are then needed to change the pH (Alitalo et al., 2012). It is expected that this should also apply to digestates. Results of past studies have shown that small quantities of chemicals can improve ammonia stripping without strong bases like sodium hydroxide (NaOH). Lime (Ca(OH)2) can be used to increase the pH to values of 8.5 to 11.5, converting the ammonium hydroxide ions into ammonia gas (Alitalo et al., 2012). Zeng and Xiaomei (2012) confirms this and also suggests that it will be preferable to carry out liming before ammonia stripping. This will not only increase the pH but it will be beneficial to the ammonia stripping process.

The USEPA (2000) essentially describes two variations of ammonia stripping towers, cross-flow and counter current. Solvent gas (air) enters a cross-flow tower along the entire depth of fill and flows through the packing, as the alkaline wastewater is pumped to the top of a packed tower. The liquid fraction after separation could be used since it is alkaline. On the other hand, a counter current tower draws air through openings at the bottom, as wastewater is pumped to the top of a packed tower. Free ammonia can be stripped from falling water droplets into the air stream. Scaling in stripping towers is often reported as a problem when a concentrated nitrogen fertilizer has to be produced from the liquid fraction of manure in which the pH has been modified by chemical addition (Ippersiel et al., 2012). These scales reduce the mass transfer of ammonia. Also, the low temperatures in stripping towers lessen the removal efficiencies of ammonia by air stripping. Existing literature shows that increasing temperatures and air flow rate improve the efficiency of stripping.

The USEPA (2000) outlines numerous advantages and disadvantages of air stripping. The advantages include its relatively simple operation, the unneeded backwash or regeneration, and the creation of a controlled process for ammonia removals. The negativities will be the high maintenance and power requirements due to water re-pumping, the scale removal which induces lots of pilot tests, its disability to work in freezing conditions, and finally the none removal of nitrite and organic nitrogen, air and noise pollution problems. Figure 1 shows a schematic representation of a stripping system. It describes how ammonia can be stripped from the liquid fraction of biologically treated manure. It is expected that this would also apply to the liquid fraction of digestates after separation and the liquid fraction of digestates after separation and filtration.

2.5.2 AIR SCRUBBER

Melse and Timmerman (2009) report that one-fourth of ammonium which is a form of nitrogen in liquid manure ends up being emitted into the atmosphere as ammonia gas. It can be concluded that technologies to decrease the emission of ammonia should be used more often in locations susceptible to ammonia emissions. Air scrubbing is an efficient technology that can be used to partially overcome the aforementioned problem. Besides ammonia reduction, air scrubbers also aid in the reduction of other environmental emissions such as odour and particulate matter (dust) from livestock production.

An air scrubber usually is a packed-bed reactor filled with an inert, inorganic packing material intermittently sprayed with water to keep it wet (Melse et al., 2009). Exhaust air driven through a scrubber results in a contact between air and water, enabling a mass transfer from gas to liquid phase. Air scrubbers were initially aimed at reducing ammonia emissions. However, they also reduce particulate matter and odour emissions to the atmosphere.

illustration not visible in this excerpt

Figure 1.Schematic representation of a stripping system

1. Stripping tower. 2. Blower. 3. Flow meter. 4. Slurry. 5. Thermostatic bath. 6. Air flow. 7. Liquid distributer. 8. Liquid flow. 9. Effluent storage tank. 10. Ammonia wet washer. Source (Alitalo et al., 2012)

Fu et al. (2011) suggests that acid air scrubbers do not only remove ammonia but they also remove other soluble gases in the air. High ammonia removal efficiencies of about 90 % to 99 % can be obtained if a well-designed acid scrubber operates under the most optimal conditions. Melse and Timmerman (2009) report that two main types of scrubbers have been generally applied: acid scrubbers and bio-scrubbers/bio-trickling filters. Acid scrubbers are based on the entrapment of ammonia in acid liquid which is re-circulated over a packed bed and frequently discharged. This results in a concentrated ammonium salt solution. According to Melse et al. (2009), sulphuric acid is commonly used as a highly efficient ammonia trap in acid air scrubbers resulting in the production of ammonium sulphate solution. They suggest that sulphuric acid can be applied at pH levels between 2 and 4. The disposal of the effluent from an acid scrubber according to Fu et al. (2011) is a major issue which limits the application of acid scrubbers in the pig and poultry industry for example.

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