Biological Waste Management. In-Vessel Composting and Anaerobic Digestion

TABLE OF CONTENTS

ABSTRACT

INTRODUCTION

BIOLOGICAL TREATMENT OF WASTE

COMPARISON BETWEEN IN-VESSEL COMPOSTING AND

POTENTIAL FEEDSTOCK ANALYSIS

DISCUSSION

FINANCIAL CONSIDERATIONS

CONCLUSION

REFERENCES

ABSTRACT

As the human population increases waste generation with its attending impacts on health and emission also increases, hence the need for legislation to guide waste disposal and treatment and discourage waste to landfills.

Waste can be treated biologically amongst other treatment methods in the presence or absence of Oxygen both giving useful end products, therefore, making waste not waste after all but resource(s) which when processed with the most efficient treatment method yields an acceptable material than can be reused. This report is an evaluation of available wastes treatment methods for food production companies in the United Kingdom. This can also be replicated in other parts of the world. In-Vessel Composting and Anaerobic Digestion are two biological ways by which food waste can be industrially treated giving importantly useful end products.

However, as a food production company whose aim above all is to treat waste, In-vessel Composting is the most appropriate treatment method for heterogeneous waste materials. In-vessel composting is also flexible, it can be combined with other waste treatment methods as the expected waste feedstocks increases and your aim for waste treatment changes or expands.

INTRODUCTION

Waste management, a cycle of collection, storage and treatment (disposal) of waste is controlled by laws and regulations from regions, Nations and International levels. The European directive 2008/98/EC gives principles for effective waste management, waste hierarchy and also when waste ceases to be waste (End-of-waste) (EC,2012) likewise the Waste (Scotland) Regulations 2012, an amendment of the waste management licensing (separately collected waste) and it also gave conditions as regards animal by-product and derived products (NetRegs, 2012). All these regulations amongst other things are aimed at protecting the health of people and reducing waste to landfill and as a food production company, waste management and treatment should be of great importance to you; it helps to increase your profit and market value.

Waste can be treated in different ways; mechanical, biological, thermal and physical treatments are some of the ways by which waste can be treated. However, as a food production company, the biological treatment of waste chief of which composting (In-Vessel) and Anaerobic digestion would be the most suitable going by the expected feedstocks (municipal and agricultural wastes). The aims of this report are therefore to analyse the two types of most suitable biological treatment of waste and decide which is the best for your company based on the analysis carried out.

BIOLOGICAL TREATMENT OF WASTE

IN-VESSEL COMPOSTING (IVC)

In-Vessel Composting (IVC) is a process that uses forced aeration with or without mechanical stirring to treat compostable materials in an enclosed (drums, silo or other containers) system and eventually produce compost. The compost is produced under uniform temperature and moisture conditioned and controlled odour (Aslam, 2007). IVC is very effective as it can be used to treat waste mixtures containing both food and garden wastes in a controlled environment with the ability to monitor the temperature and control it (Wrap, 2012). Setting up an IVC requires considering criteria such as existing land use and users, proximity to receptors and transport infrastructures. Its process is about 3 stages and can be used all year round.

BSI PAS 100 is the British Standard Institute certification for composting, this standard helps to identify and distinguish safe, reliable and effective compost and it’s a requirement for waste treatment (wrap, 2012).

REQUIREMENTS FOR IN-VESSEL COMPOSTING

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Table 1 shows the requirements for In-vessel composting. Source: (EPA, 2012)

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Figure 1 showing the In-vessel Composting process. (Source: Haaren, 2009)

ANAEROBIC DIGESTION (AD)

Anaerobic Digestion (AD) is a process whereby in the absence of oxygen micro-organisms “naturally” breakdown organic matters to produce biogas (a mixture of carbon dioxide and methane) and digestate (Nitrogen-rich fertilizer); biogas is used to produce energy which could be used for combined heat and power (CHP) while the digestate can be used for agricultural production (DEFRA, 2012). Anaerobic Digestion can either be wet (with moisture content more than 85%) or dry (moisture content less than 80%) however, economically there’s little to choose between both.

The process of AD can be divided into four stages viz: HYDROLYSIS, acidogenesis, ACETOGENESIS and METHANOGENESIS. There are factors that control the conversion of waste to gas and some include waste characteristics, dilution of waste (concentration), foreign materials, toxic materials, temperature and ratio of micro-organism. AD has a standardization of BSI PAS 110.

REQUIREMENTS FOR ANAEROBIC DIGESTION

Table 2: Requirements for Anaerobic Digestion

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Figure 2 showing the process of anaerobic digestion of waste.

(Source: Shefali, 2002)

COMPARISON BETWEEN IN-VESSEL COMPOSTING AND

ANAEROBIC DIGESTION

ADVANTAGES OF IN-VESSEL COMPOSTING OVER OTHER FORMS OF COMPOSTING

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Table 1 showing feedstock Analysis for the proposed treatment plan

DISCUSSION

The proposed feedstock for the planned waste treatment plant was analysed for important waste treatment parameters, Moisture Content, % dry solid and C: N ratio amongst others and the analysis showed that the feedstock can be biologically treated using one of In-vessel composting (IVC) or Anaerobic Digestion (AD) (Forgie et al, 2004). The result of the analysis showed that the C: N ratio of the feedstock calculated in tonnes/year is 23.64 which makes the feedstock good for both IVC and AD. Also, the Moisture Content was analysed to be 73.35% a range above the optimum moisture content range for IVC which is 60 – 65% but falls in AD (wet process) range (Karnchanawong, 2011); however, Abdelhadi et al., (2013) explains further that %MC of feedstock is greatly affected by waste materials characteristics. Although Anaerobic Digestion (dry process) could appear like a good treatment method based on the analysis, In-vessels composting in my judgment is still the best method to use.

Moisture content is important because directly, it influences oxygen supply and microbes activities; 78% MC can result in system failure due to leachate production. However, this can be panacea by splitting the composting process into two viz: the feeding stage when the feedstock is added to the reactor and the curing stage where the composting process is continued till it turned to compost i.e. the composting process should be done in batches, controlling the amount of waste in the feeding batch) composting is possible for high MC% feedstock so far there’s enough air to satisfy the oxygen requirement of the microbes in the compost pile (Uao et al, 2008). Increasing the tonnage of materials such as straws and papers which could serve as bulking agents would help better compost the mixture as bulky and fibrous materials can readily absorb moisture and still maintain their structures (Bijaya, 2005).

The aim of your company is to treat waste (starting with your waste which is food waste) thereby reducing the cost of production and also gather up profits, this aim would be best achieved using IVC as AD doesn’t give a 100% sustainable waste management solution as it doesn’t give a complete biodegradable waste. Compost is an agronomically accepted useful fertilizer and soil fertility enhancer digestate, on the other hand, is relatively new to the market and will thus require more awareness for it to be marketable.

Although AD produces biogas that could be used for CHP (Combined Heat and Power), energy from waste still constitutes a very small fraction of the UK total energy production likewise the heat. Ready make market must be sourced for Anaerobic Digestion’s end product unlike composting that has market readily available.

FINANCIAL CONSIDERATIONS

Setting up an IVC facility is cheaper and cost-efficient when compared to AD depending on the size, feedstock and location, the capital cost would be estimated at around £1.8b and above for the expected amount of feedstock (7,150tonnes/year). Operations cost would largely be depending on staff strength, transportation, and maintenance and could averagely be around £80,000/year (Biocycle, 2011). Setting up an Anaerobic Digestion is expensive and requires more technology and operations monitoring. However, gate fees (levy on each waste received at the treatment facility) should be charged to help reduce the cost of running the facility, according to WRAP (9th July 2012), gate fee in the range of £35 - £60 could be charged per tonne.

CONCLUSION

As a food production company high waste disposal cost would negative impacts on your profits but efficient waste treatment can serve as an image booster. It is therefore important not to choose a waste treatment based only on personal likeness for a particular system but to also consider the effects of important analysis parameters and the prevailing market and finances.

Having a plan for uncertainties is a strategy in business which I believe your company also practices. While AD is most efficient when used to process specific source-separated feedstock as too much garden mix in feedstock reduces yield of biogas and lignin requires oxygen to breakdown, IVC has a more flexible system; it can be used all year round making it suitable when feedstock changes or expansion is made for the system to accommodate more waste.

The feedstock analysis clearly showed that both preferred waste treatment methods could be used for the feedstock, but In-vessel composting is more suitable for your company. The information and knowledge gathered in this work will be of great use to companies interested in waste treatments and also researchers on ways to improve on both methods.

APPENDIX

With the scarcity of resources becoming evident and potential dearth in view, sustainability is fast becoming the bedrock of our society and development, there is, therefore, we need to see waste from a different definition, not as wastes but as resources. Waste is a resource that is not safely recycled back for use (ZerowasteAmerica, 2010). Safely recycled back means what used to be waste now undergoes treatment to become a waste, it is therefore until all available treatment process has failed that material becomes a waste.

Waste is produced as a result of human activities, all businesses produce waste and as a food production company, waste management, and treatment should be of great importance to you; it helps to increase your profit and market value, likewise, as environmental regulations get tough worldwide efficient waste treatment becomes a task (Shulin, 2007).

STAGES OF IN-VESSEL COMPOSTING

(1) STAGE 1: Involves the collection of feedstock materials and shredding them in an enclosed area (dirty area). The process takes about 7 - 21 days.
(2) STAGE 2: at this stage, it is ensured that the composting mass reaches the necessary temperature. The process takes also about 7 - 21 days.
(3) STAGE 3: compost is left for between 10 -14 days to mature and achieve stabilization (ZerowasteScotland, 2013).

WET AND DRY ANAEROBIC DIGESTION

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STAGES OF ANAEROBIC DIGESTION

1. HYDROLYSIS: breaking down of organic compounds to sugar and amino acids.
2. ACIDOGENESIS: conversion of soluble sugar, fat and amino acids to CO2, alcohol, ammonia, organic acids and hydrogen.
3. ACETOGENESIS: conversion of the above to acetic acid, CO2 and hydrogen.
4. METHANOGENESIS: conversion of acetic acid, CO2 and hydrogen to methane and CO2.

FEEDSTOCK ANALYSIS

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ANALYSIS RESULT TABLE

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A = MUNICIPAL SOURCE-SEPARATED FOOD WASTE

B= MUNICIPAL SEWAGE PLAND BIO-SOLID

C= DIARY CATTLE MANURE

D= BROILER CHICKEN MANURE

E= VEGETABLE WASTE

F= POTATO PEELINGS

G= ANIMAL BY-PRODUCTS

REFERENCES

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