Renewable Energy from Food Residuals

Food residuals are an untapped renewable energy source that mostly ends up rotting in landfills, thereby releasing greenhouse gases into the atmosphere. Food residuals are difficult to treat or recycle since it contains high levels of sodium salt and moisture, and is mixed with other waste during collection. Major generators of food wastes include hotels, restaurants, supermarkets, residential blocks, cafeterias, airline caterers, food processing industries, etc.

In United States, food scraps is the third largest waste stream after paper and yard waste. Around 12.7 percent of the total municipal solid waste (MSW) generated in the year 2008 was food scraps that amounted to about 32 million tons. According to EPA, about 31 million tons of food waste was thrown away into landfills or incinerators in 2008. As far as United Kingdom is concerned, households throw away 8.3 million tons of food each year. These statistics are an indication of tremendous amount of food waste generated all over the world.

The proportion of food residuals in municipal waste stream is gradually increasing and hence a proper food waste management strategy needs to be devised to ensure its eco-friendly and sustainable disposal. Currently, only about 3 percent of food waste is recycled throughout U.S., mainly through composting. Composting provides an alternative to landfill disposal of food waste, however it requires large areas of land, produces volatile organic compounds and consumes energy. Consequently, there is an urgent need to explore better recycling alternatives.

Anaerobic digestion has been successfully used in several European and Asian countries to stabilize food wastes, and to provide beneficial end-products. Sweden, Austria, Denmark, Germany and England have led the way in developing new advanced biogas technologies and setting up new projects for conversion of food waste into energy.

Anaerobic Digestion of Food Waste

Anaerobic digestion is the most important method for the treatment of organic waste, such as food residuals, because of its techno-economic viability and environmental sustainability. The use of anaerobic digestion technology generates biogas and preserves the nutrients which are recycled back to the agricultural land in the form of slurry or solid fertilizer.

The relevance of biogas technology lies in the fact that it makes the best possible use of various organic wastes as a renewable source of clean energy. A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. Thus, anaerobic digestion of food waste can lead to climate change mitigation, economic benefits and landfill diversion opportunities.

Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer. Food waste can either be used as a single substrate in a biogas plant, or can be co-digested with organic wastes like cow manure, poultry litter, sewage, crop residues, slaughterhouse wastes, etc.

A Typical Energy Conversion Plant

The feedstock for the food waste-to-energy plant includes leftover food, vegetable refuse, stale cooked and uncooked food, meat, teabags, napkins, extracted tea powder, milk products, etc. Raw waste is shredded to reduce to its particle size to less than 12 mm. The primary aim of shredding is to produce a uniform feed and reduce plant “down-time” due to pipe blockages by large food particles. It also improves mechanical action and digestibility and enables easy removal of any plastic bags or cling-film from waste.

Fresh waste and re-circulated digestate (or digested food waste) are mixed in a mixing tank. The digestate is added to adjust the solids content of the incoming waste stream from 20 to 25 percent (in the incoming waste) to the desired solids content of the waste stream entering the digestion system (10 to 12 percent total solids). The homogenized waste stream is pumped into the feeding tank, from which the anaerobic digestion system is continuously fed. Feeding tank also acts as a pre-digester and subjected to heat at 55º to 60º C to eliminate pathogens and to facilitate the growth of thermophilic microbes for faster degradation of waste.

From the predigestor tank, the slurry enters the main digester where it undergoes anaerobic degradation by a consortium of Archaebacteria belonging to Methanococcus group. The anaerobic digester is a CSTR reactor having average retention time of 15 to 20 days. The digester is operated in the mesophilic temperature range (33º to 38°C), with heating carried out within the digester. Food waste is highly biodegradable and has much higher volatile solids destruction rate (86 to 90 percent) than biosolids or livestock manure. As per conservative estimates, each ton of food waste produces 150 to 200 m3 of biogas, depending on reactor design, process conditions, waste composition, etc.

Biogas contains significant amount of hydrogen sulfide (H2S) gas that needs to be stripped off due to its corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria that oxidizes H2S into elemental sulfur. The biogas produced as a result of anaerobic digestion of waste is sent to a gas holder for temporary storage. Biogas is eventually used in a combined heat and power (CHP) unit for its conversion into thermal and electrical energy in a co­generation power station of suitable capacity. The exhaust gases from the CHP unit are used for meeting process heat requirements.

The digested substrate leaving the reactor is rich in nutrients like nitrogen, potassium and phosphorus which are beneficial for plants as well as soil. The digested slurry is dewatered in a series of screw presses to remove the moisture from slurry. Solar drying and additives are used to enhance the market value and handling characteristics of the fertilizer.

Diverting Food from Landfills

Food residuals are one of the single largest constituents of municipal solid waste stream. Diversion of food waste from landfills can provide significant contribution towards climate change mitigation, apart from generating revenues and creating employment opportunities. Rising energy prices and increasing environmental pollution makes it more important to harness renewable energy from food scraps.

Anaerobic digestion technology is widely available worldwide and successful projects are already in place in several European as well as Asian countries that makes it imperative on waste generators and environmental agencies to root for a sustainable food waste management system.

Biogas from Crop Wastes: European Perspectives

Most, if not all of Europe has a suitable climate for biogas production. The specific type of system depends on the regional climate. Regions with harsher winters may rely more on animal waste and other readily available materials compared to warmer climates, which may have access to more crop waste or organic material.

Regardless of suitability, European opinions vary on the most ethical and appropriate materials to use for biogas production. Multiple proponents argue biogas production should be limited to waste materials derived from crops and animals, while others claim crops should be grown with the intention of being used for biogas production.

Biogas Production From Crops

Europeans in favor of biogas production from crops argue the crops improve the quality of the soil. Additionally, they point to the fact that biogas is a renewable energy resource compared to fossil fuels. Crops can be rotated in fields and grown year after year as a sustainable source of fuel.

Extra crops can also improve air quality. Plants respire carbon dioxide and can help reduce harmful greenhouse gasses in the air which contribute to global climate change.

Biogas crops can also improve water quality because of plant absorption. Crops grown in otherwise open fields reduce the volume of water runoff which makes it to lakes, streams and rivers. The flow of water and harmful pollutants is impeded by the plants and eventually absorbed into the soil, where it is purified.

Urban residents can also contribute to biogas production by growing rooftop or vertical gardens in their homes. Waste from tomatoes, beans and other vegetables is an excellent source of biogas material. Residents will benefit from improved air quality and improved water quality as well by reducing runoff.

Proponents of biogas production from crops aren’t against using organic waste material for biogas production in addition to crop material. They believe crops offer another means of using more sustainable energy resources.

Biogas Production From Waste Materials Only

Opponents to growing crops for biogas argue the crops used for biogas production degrade soil quality, making it less efficient for growing crops for human consumption. They also argue the overall emissions from biogas production from crops will be higher compared to fossil fuels.

Growing crops can be a labor-intensive process. Land must be cleared, fertilized and then seeded. While crops are growing, pesticides and additional fertilizers may be used to promote crop growth and decrease losses from pests. Excess chemicals can run off of fields and degrade the water quality of streams, lakes and rivers and kill off marine life.

Once crops reach maturity, they must be harvested and processed to be used for biogas material. Biogas is less efficient compared to fossil fuels, which means it requires more material to yield the same amount of energy. Opponents argue that when the entire supply chain is evaluated, biogas from crops creates higher rates of emissions and is more harmful to the environment.

Agricultural residues, such as rice straw, are an important carbon source for anaerobic digestion

The supply chain for biogas from agricultural waste materials is more efficient compared to crop materials. Regardless of whether or not the organic waste is reused, it must be disposed of appropriately to prevent any detrimental environmental impacts. When the waste material is then used for biogas production, it creates an economical means of generating useful electricity from material which would otherwise be disposed of.

Rural farms which are further away from the electric grid can create their own sources of energy through biogas production from waste material as well. The cost of the energy will be less expensive and more eco-friendly as it doesn’t have the associated transportation costs.

Although perspectives differ on the type of materials which should be used for biogas production, both sides agree biogas offers an environmentally friendly and sustainable alternative to using fossil fuels.

Miscanthus: Reducing the Establishment Costs

Miscanthus-Elephant-GrassMiscanthus has been lauded as a dynamic high potential biomass crop for some time now due to its high yields, low input requirements and perennial nature. Miscanthus is commonly used as a biomass fuel to produce heat and electricity through combustion, but studies have found that miscanthus can produce similar biogas yields to maize when harvested at certain times of the year.  Miscanthus is a C4 grass closely related to maize and sugarcane, it can grow to heights of three metres in a single growing season.

High Establishment Costs

However, high establishment costs have impeded the popularity of the crop. High establishment costs of miscanthus are as a result of the sterile nature of the crop, which means that miscanthus cannot be propagated from seed and instead must be propagated from vegetative material. The vegetative material commonly used is taken from the root structure known as rhizomes; rhizome harvesting is a laborious process and when combined with low multiplication rates, results in a high cost for miscanthus rhizomes. The current figure based on Irish figures is €1,900 ha for rhizomes.

Promising Breakthrough

Research conducted in Teagasc Oak Park Carlow Ireland, suggests that there may be a cost effective of method of propagating miscanthus by using the stem as the vegetative material rather than having to dig up expensive rhizomes. The system has been proven in a field setting over two growing seasons and plants have been shown to be perennial.

A prototype planter suitable for commercial up scaling has been developed to sow stem segments of miscanthus. Initial costs are predicted at €130 ha for plant material. The image below shows the initial stem that was planted in a field setting and the shoots, roots, and rhizome developed by the stem at the end of the first growing season.

miscanthus-stem

Feedstock for AD Plants

Switching from maize to miscanthus as a feedstock for anaerobic digestion plants would increase profitability and boost the GHG abatement credentials of the systems. Miscanthus is a perennial crop which would provide a harvest every year once established for 20 years in a row without having to be replanted compared to maize which is replanted every year. This would provide an obvious economic saving as well as allowing carbon sequestration in the undisturbed soil.

There would be further GHG savings from the reduced diesel consumption required for the single planting as opposed to carrying out heavy seedbed cultivation each year for maize. Miscanthus harvested as an AD feedstock would also alleviate soil compaction problems associated with maize production through an earlier harvest in more favourable conditions.

Future Perspectives

Miscanthus is a nutrient efficient crop due to nutrient cycling. With the onset of senescence nutrients in the stem are transferred back to the rhizome and over-wintered for the following year’s growth. However the optimum date to harvest biomass to produce biogas is before senescence. This would mean that a significant proportion of the plants nutrient stores would be removed which would need to be replaced. Fertiliser in the form of digestate generated from a biogas plant could be land spread to bridge nutrient deficiencies. However additional more readily available chemical N fertiliser may have to be applied.

Some work at Oak Park on September harvested miscanthus crops has seen significant responses from a range of N application rates. With dwindling subsidies to support anaerobic digestion finding a low cost perennial high yielding feedstock could be key to ensuring economic viability.

Description of a Biogas Power Plant

A biogas plant is a decentralized energy system, which can lead to self-sufficiency in heat and power needs, and at the same time reduces environmental pollution. The components of a modern biogas (or anaerobic digestion) plant include: manure collection, anaerobic digester, effluent treatment, biogas storage, and biogas use/electricity generating equipment.

Working of a Biogas Plant

The fresh animal manure is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digesters where stabilization of organic waste takes place.

In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.

Biogas Cleanup

Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.

Utilization of Biogas

Biogas is dried and vented into a CHP unit to a generator to produce electricity and heat. The size of the CHP system depends on the amount of biogas produced daily.

Treatment of Digestate

The digested substrate is passed through screw presses for dewatering and then subjected to solar drying and conditioning to give high-quality organic fertilizer.  The press water is treated in an effluent treatment plant based on activated sludge process which consists of an aeration tank and a secondary clarifier. The treated wastewater is recycled to meet in-house plant requirements.

Monitoring of Environmental Parameters

A chemical laboratory is necessary to continuously monitor important environmental parameters such as BOD, COD, VFA, pH, ammonia, C:N ratio at different locations for efficient and proper functioning of the process.

Control System

The continuous monitoring of the biogas plant is achieved by using a remote control system such as Supervisory Control and Data Acquisition (SCADA) system. This remote system facilitates immediate feedback and adjustment, which can result in energy savings.

Importance of Biomass Energy

Biomass energy has rapidly become a vital part of the global renewable energy mix and account for an ever-growing share of electric capacity added worldwide. Renewable energy supplies around one-fifth of the final energy consumption worldwide, counting traditional biomass, large hydropower, and “new” renewables (small hydro, modern biomass, wind, solar, geothermal, and biofuels).

Traditional biomass, primarily for cooking and heating, represents about 13 percent and is growing slowly or even declining in some regions as biomass is used more efficiently or replaced by more modern energy forms. Some of the recent predictions suggest that biomass energy is likely to make up one third of the total world energy mix by 2050. Infact, biofuel provides around 3% of the world’s fuel for transport.

Biomass energy resources are readily available in rural and urban areas of all countries. Biomass-based industries can foster rural development, provide employment opportunities and promote biomass re-growth through sustainable land management practices.

The negative aspects of traditional biomass utilization in developing countries can be mitigated by promotion of modern waste-to-energy technologies which provide solid, liquid and gaseous fuels as well as electricity. Biomass wastes encompass a wide array of materials derived from agricultural, agro-industrial, and timber residues, as well as municipal and industrial wastes.

The most common technique for producing both heat and electrical energy from biomass wastes is direct combustion. Thermal efficiencies as high as 80 – 90% can be achieved by advanced gasification technology with greatly reduced atmospheric emissions.

Combined heat and power (CHP) systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity. Biochemical processes, like anaerobic digestion and sanitary landfills, can also produce clean energy in the form of biogas and producer gas which can be converted to power and heat using a gas engine.

Advantages of Biomass Energy

Bioenergy systems offer significant possibilities for reducing greenhouse gas emissions due to their immense potential to replace fossil fuels in energy production. Biomass reduces emissions and enhances carbon sequestration since short-rotation crops or forests established on abandoned agricultural land accumulate carbon in the soil.

Bioenergy usually provides an irreversible mitigation effect by reducing carbon dioxide at source, but it may emit more carbon per unit of energy than fossil fuels unless biomass fuels are produced unsustainably.

Biomass can play a major role in reducing the reliance on fossil fuels by making use of thermochemical conversion technologies. In addition, the increased utilization of biomass-based fuels will be instrumental in safeguarding the environment, generation of new job opportunities, sustainable development and health improvements in rural areas.

The development of efficient biomass handling technology, improvement of agro-forestry systems and establishment of small and large-scale biomass-based power plants can play a major role in rural development. Biomass energy could also aid in modernizing the agricultural economy.

Consistent and reliable supply of biomass is crucial for any biomass project

When compared with wind and solar energy, biomass power plants are able to provide crucial, reliable baseload generation. Biomass plants provide fuel diversity, which protects communities from volatile fossil fuels. Since biomass energy uses domestically-produced fuels, biomass power greatly reduces our dependence on foreign energy sources and increases national energy security.

A large amount of energy is expended in the cultivation and processing of crops like sugarcane, coconut, and rice which can met by utilizing energy-rich residues for electricity production.

The integration of biomass-fueled gasifiers in coal-fired power stations would be advantageous in terms of improved flexibility in response to fluctuations in biomass availability and lower investment costs. The growth of the bioenergy industry can also be achieved by laying more stress on green power marketing.

Biogas Plants at Akshaya Patra Kitchens

Akshaya-Patra-Kitchen-BioGasThe Akshaya Patra Foundation, a not-for-profit organization, is focused on addressing two of the most important challenges in India – hunger and education. Established in year 2000, the Foundation began its work by providing quality mid-day meals to 1500 children in 5 schools in Bangalore with the understanding that the meal would attract children to schools, after which it would be easier to retain them and focus on their holistic development. 14 years later, the Foundation has expanded its footprint to cover over 1.4 million children in 10 states and 24 locations across India.

The Foundation has centralised, automated kitchens that can cook close to 6,000 kilos of rice, 4.5 to 5 tonnes of vegetables and 6,000 litres of sambar, in only 4 hours. In order to make sustainable use of organic waste generated in their kitchens, Akshaya Patra Foundation has set up anaerobic digestion plants to produce biogas which is then used as a cooking fuel. The primary equipment used in the biogas plant includes size reduction equipment, feed preparation tank for hydrolysis of waste stream, anaerobic digester, H2S scrubber and biogas holder.

Working Principle

Vegetable peels, rejects and cooked food waste are shredded and soaked with cooked rice water (also known as ganji) in a feed preparation tank for preparation of homogeneous slurry and fermentative intermediates. The hydrolyzed products are then utilized by the microbial culture, anaerobically in the next stage. This pre-digestion step enables faster and better digestion of organics, making our process highly efficient.

The hydrolyzed organic slurry is fed to the anaerobic digester, exclusively for the high rate biomethanation of organic substrates like food waste. The digester is equipped with slurry distribution mechanism for uniform distribution of slurry over the bacterial culture.

Optimum solids are retained in the digester to maintain the required food-to-microorganism ratio in the digester with the help of a unique baffle arrangement. Mechanical slurry mixing and gas mixing provisions are also included in the AD design to felicitate maximum degradation of organic material for efficient biogas production.

After trapping moisture and scrubbing off hydrogen sulphide from the biogas, it is collected in a gas-holder and a pressurized gas tank. This biogas is piped to the kitchen to be used as a cooking fuel, replacing LPG.

Basic Design Data and Performance Projections

Waste handling capacity 1 ton per day cooked and uncooked food waste with 1 ton per day ganji water

Input Parameters                      

Amount of solid organic waste 1000 Kg/day
Amount of organic wastewater ~ 1000 liters/day ganji (cooked rice water)

Biogas Production

Biogas production ~ 120 – 135 m3/day

Output Parameters

Equivalent LPG to replace 50 – 55 Kg/day (> 2.5 commercial LPG cylinders)
Fertilizer (digested leachate) ~ 1500 – 2000 liters/day

Major Benefits

Modern biogas installations are providing Akshaya Patra, an ideal platform for managing organic waste on a daily basis. The major benefits are:

  • Solid waste disposal at kitchen site avoiding waste management costs
  • Immediate waste processing overcomes problems of flies, mosquitos etc.
  • Avoiding instances when the municipality does not pick up waste, creating nuisance, smell, spillage etc.
  • Anaerobic digestion of Ganji water instead of directly treating it in ETP, therefore reducing organic load on the ETPs and also contributing to additional biogas production.

The decentralized model of biogas based waste-to-energy plants at Akshaya Patra kitchens ensure waste destruction at source and also reduce the cost incurred by municipalities on waste collection and disposal.

akshayapatra-kitchen

An on-site system, converting food and vegetable waste into green energy is improving our operations and profits by delivering the heat needed to replace cooking LPG while supplying a rich liquid fertilizer as a by-product.  Replacement of fossil fuel with LPG highlights our organization’s commitment towards sustainable development and environment protection.

The typical ROI of a plug and play system (without considering waste disposal costs, subsidies and tax benifts) is around three years.

Future Plans

Our future strategy for kitchen-based biogas plant revolves around two major points:

  • Utilization of surplus biogas – After consumption of biogas for cooking purposes, Akshaya Patra will consider utilizing surplus biogas for other thermal applications. Additional biogas may be used to heat water before boiler operations, thereby reducing our briquette consumption.
  • Digested slurry to be used as a fertilizer – the digested slurry from biogas plant is a good soil amendment for landscaping purposes and we plan to use it in order to reduce the consumption of water for irrigation as well as consumption of chemical fertilizers.

Pet Waste Management in UK – Prospects and Challenges

pet-wastesPet waste is a growing public health and environmental risk. According to a report commissioned by the Pet Food Manufacturers’ Association, 13 million UK households (45%) keep pets of some kind.  Cats and dogs are each kept by 8.5 million households (these numbers are not additive, as some will of course keep both).

Can those of us who want both the joys of animal companionship and waste minimisation, find ways to cut down, or better manage, the huge amount of pet waste generated in the UK every year? With so many cats and dogs in the UK, pet waste must represent a significant mass of organic matter within the residual waste stream.

Does this waste represent a floater in the residual waste stream by necessity—due to inherently unpleasant and possibly dangerous characteristics of the waste—or is it only there out of convention and squeamishness?

I’ve written before about the relationship between waste management and squeamishness, and talking about faeces really brings the point home. There are some undoubtedly nasty pathogens present in pet faeces, notably the parasites Toxocariasis and Toxoplasmosis. But might these be safely killed off by the temperatures reached in anaerobic digestion (AD)? If so, provided any litter and bags were made of organic matter, might pet waste be collected along with food waste?

I began by contacting a local authority waste officer, but was told that no one had asked this question before, and that I might be better off talking to AD plant operators. This I did, but most seemed similarly baffled by my query.  However, one mentioned that AD digestate goes through a pasteurisation process, where it is heated to a temperature of 70oC for one hour, in order to make it safe for land application. I also attempted to contact some technical specialists in the field, but to no avail.

There are some theoretical indications that this pasteurisation should be sufficient. Hanna Mizgajska-Wiktor and Shoji Uga’s  essay Exposure and Environmental Contamination states: “Anaerobic waste treatment kills Toxocara spp. eggs at temperatures in excess of 45oC”, well below the 70oC mentioned by my operator. The susceptibility of Toxoplasma to heat is less clear, although numerous internet sources suggest this can be killed in meat by cooking at 66oC. So far, then, I haven’t confirmed or falsified my initial inkling, and so the collection of pet waste in the municipal organic stream remains a theoretical possibility.

Motivated dog owners  can already turn their pet’s waste into a resource within their own home. The website London Worms explains how you can turn your dog’s poo into rich and useful vermicompost, although it warns that the results will only be suitable for use on non-edible plants.

Foul Pay

Household pet droppings may still be largely fated for disposal, but even when binned this waste is at least moving through proper waste management channels.  Unfortunately, not all pet poo is binned, and we have real data measuring public perceptions of the disamenity resulting from dog fouling. For most, the presence of this unwelcome waste in our streets, parks and footpaths is of much higher concern than its diversion from landfill.

A 2011 Defra-funded study on local residents’ willingness-to-pay — via an increase in council tax — for improvements across a range of environmental factors found that dog fouling was the third most important issue out of the presented range (with litter and fly-tipping taking first and second place). Surveys were conducted in inner-city, suburban and rural/semi-rural areas around London, Manchester and Coventry.

In order to move from the current level of dog fouling to the best possible scenario, it was found that inner-city residents would on average be willing to pay £8.87 per month, suburban residents £7.79 per month, and rural residents £2.72. Combining these figures with population statistics allows us to place a disamenity value on dog fouling. National statistics only allow for an urban-rural split, but based on a 2012 Defra rurality study which found that 18.9% of the population lives in rural areas, we can calculate that across England we would collectively be willing to pay £462m per year to achieve best case scenario improvements in dog fouling.

This somewhat crude calculation gives an indication of the perceived disamenity of dog fouling. Presenting the matter in terms such as these may allow economically minded policy makers a means of engaging with this important street scene issue and evaluating the costs and benefits of interventions.

Food for Thought

Let’s wash our hands of poo (with plenty of soap and warm water) and look to the other end of the pet waste problem. According to a report published by WRAP, the UK uses around 75,000 tonnes of primary packaging annually. This holds 1,263,000 tonnes of wet and dry cat and dog food, of which 9,000 uneaten tonnes are thrown away. Although this wasted food constitutes less than 1% of the total sold (if only we were as careful with food for human consumption) the estimated cost to the consumer is still £21m a year.

WRAP examined a number of designs intended to cut to down on the amounts of both pet food and packaging thrown away. A major problem with packaging design is the need to account for portion sizes, which vary from animal to animal and change depending on age and level of activity. Single serve packaging may actually lead to regular food wastage if the portion provided is too big for a particular pet; indeed, this is a problem I am experiencing with my own cat, whose appetite seems to fluctuate wildly. Re-sealable packaging that allows owners to dish out meals in accordance with the changing appetites of their pets is therefore preferable.

The material that packaging is made of is also significant: for example, relatively heavy tins are recyclable, whereas lightweight plasticised plastic foil packets are not. Pet food and its packaging can be pushed up the waste hierarchy by simply choosing a recyclable and resealable container which will allow them to adequately provide for the appetite of their pet. However, these issues are likely to be given less weight compared with health, convenience and cost in the minds of most householders. The onus has to be on manufacturers to develop packaging which is both low cost and easily recyclable.

Love pets, hate waste?

People love animals, but are rather less keen to engage with pets as an environmental issue. Leaving aside questions of whether it is sustainable for so many of us to have pets at all, there are clearly ways in which we can reduce their impact. The convenience of single serving pouches of pet food seems to win out over more recyclable and waste-avoiding alternatives, although pet owners might be willing to change their choices if presented with a better option.

While worrying about recovery options for cat poo might seem somewhat academic, it may be easier to tackle than dog fouling. It might even help to tackle the common psycho-social root of both issues. Cultural distaste perhaps lies behind the lack of information available on dealing with household pet waste, and the persistence of dog fouling as a street scene issue. Things were very different in Victorian London when “pure finders” earned a living by seeking out doggie doo to supply the tanning trade. But for us this kind of waste is a disagreeable fact of life which we deal with as simply and with as little thought as possible. But as a nation of animal lovers, it’s our responsibility to engage with the waste management issues our pets present.

Note: The article is being republished with the kind permission of our collaborative partner Isonomia. The original article can be viewed at this link

Trends in Utilization of Biogas

The valuable component of biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases. The proportion of methane depends on the feedstock and the efficiency of the process, with the range for methane content being 40% to 70%. Biogas is saturated and contains H2S, and the simplest use is in a boiler to produce hot water or steam.

The most common use is where the biogas fuels an internal combustion gas engine in a Combined Heat and Power (CHP) unit to produce electricity and heat. In Sweden the compressed gas is used as a vehicle fuel and there are a number of biogas filling stations for cars and buses. The gas can also be upgraded and used in gas supply networks. The use of biogas in solid oxide fuel cells is also being researched.

Biogas can be combusted directly to produce heat. In this case, there is no need to scrub the hydrogen sulphide in the biogas. Usually the process utilize dual-fuel burner and the conversion efficiency is 80 to 90%. The main components of the system are anaerobic digester, biogas holder, pressure switch, booster fan, solenoid valve, dual fuel burner and combustion air blower.

The most common method for utilization of biogas in developing countries is for cooking and lighting. Conventional gas burners and gas lamps can easily be adjusted to biogas by changing the air to gas ratio. In more industrialized countries boilers are present only in a small number of plants where biogas is used as fuel only without additional CHP. In a number of industrial applications biogas is used for steam production.

Burning biogas in a boiler is an established and reliable technology. Low demands are set on the biogas quality for this application. Pressure usually has to be around 8 to 25 mbar. Furthermore it is recommended to reduce the level of hydrogen sulphide to below 1 000 ppm, this allows to maintain the dew point around 150 °C.

CHP Applications

Biogas is the ideal fuel for generation of electric power or combined heat and power. A number of different technologies are available and applied. The most common technology for power generation is internal combustion. Engines are available in sizes from a few kilowatts up to several megawatts. Gas engines can either be SI-engines (spark ignition) or dual fuel engines. Dual fuel engines with injection of diesel (10% and up) or sometimes plant oil are very popular in smaller scales because they have good electric efficiencies up to guaranteed 43%.

The biogas pressure is turbo-charged and after-cooled and has a high compression ratio in the gas engines. The cooling tower provides cooling water for the gas engines. The main component of the system required for utilizing the technology are anaerobic digester, moisture remover, flame arrester, waste gas burner, scrubber, compressor, storage, receiver, regulator, pressure switch and switch board.

Gas turbines are an established technology in sizes above 500 kW. In recent years also small scale engines, so called micro-turbines in the range of 25 to 100kW have been successfully introduced in biogas applications. They have efficiencies comparable to small SI-engines with low emissions and allow recovery of low pressure steam which is interesting for industrial applications. Micro turbines are small, high-speed, integrated power plants that include a turbine, compressor, generator and power electronics to produce power.

New Trends

The benefit of the anaerobic treatment will depend on the improvement of the process regarding a higher biogas yield per m3 of biomass and an increase in the degree of degradation. Furthermore, the benefit of the process can be multiplied by the conversion of the effluent from the process into a valuable product. In order to improve the economical benefit of biogas production, the future trend will go to integrated concepts of different conversion processes, where biogas production will still be a significant part. In a so-called biorefinery concept, close to 100% of the biomass is converted into energy or valuable by-products, making the whole concept more economically profitable and increasing the value in terms of sustainability.

Typical layout of a modern biogas facility

One example of such biorefinery concept is the Danish Bioethanol Concept that combines the production of bioethanol from lignocellulosic biomass with biogas production of the residue stream. Another example is the combination of biogas production from manure with manure separation into a liquid and a solid fraction for separation of nutrients. One of the most promising concepts is the treatment of the liquid fraction on the farm-site in a UASB reactor while the solid fraction is transported to the centralized biogas plant where wet-oxidation can be implemented to increase the biogas yield of the fiber fraction. Integration of the wet oxidation pre-treatment of the solid fraction leads to a high degradation efficiency of the lignocellulosic solid fraction.

Biological Cleanup of Biogas

The most valuable component of biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases. However, biogas also contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. Hydrogen sulfide is oxidized into sulfur dioxide which dissolves as sulfuric acid. Sulphuric acid, even in trace amounts, can make a solution extremely acidic. Extremely acidic electrolytes dissolve metals rapidly and speed up the corrosion process.

The corrosive nature of H2S has the potential to destroy expensive biogas processing equipment. Even if there is no oxygen present, biogas can corrode metal. Hydrogen sulphide can become its own electrolyte and absorb directly onto the metal to form corrosion. If the hydrogen sulphide concentration is very low, the corrosion will be slow but will still occur due to the presence of carbon dioxide.

The obvious solution is the use of a biogas cleanup process whereby contaminants in the raw biogas stream are absorbed or scrubbed. Desulphurization of biogas can be performed by biological as well as chemical methods. Biological treatment of hydrogen sulphide typically involves passing the biogas through biologically active media. These treatments may include open bed soil filters, biofilters, fixed film bioscrubbers, suspended growth bioscrubbers and fluidized bed bioreactors.

Biological Desulphurization

The simplest method of desulphurization is the addition of oxygen or air directly into the digester or in a storage tank serving at the same time as gas holder. Thiobacilli are ubiquitous and thus systems do not require inoculation. They grow on the surface of the digestate, which offers the necessary micro-aerophilic surface and at the same time the necessary nutrients. They form yellow clusters of sulphur. Depending on the temperature, the reaction time, the amount and place of the air added the hydrogen sulphide concentration can be reduced by 95 % to less than 50 ppm.

Most of the sulphide oxidising micro-organisms belong to the family of Thiobacillus. For the microbiological oxidation of sulphide it is essential to add stoichiometric amounts of oxygen to the biogas. Depending on the concentration of hydrogen sulphide this corresponds to 2 to 6 % air in biogas. Measures of safety have to be taken to avoid overdosing of air in case of pump failures.

Biofiltration

Biofiltration is one of the most promising clean technologies for reducing emissions of malodorous gases and other pollutants into the atmosphere. In a biofiltration system, the gas stream is passed through a packed bed on which pollutant-degrading microbes are immobilized as biofilm. A biological filter combines water scrubbing and biological desulfurization. Biogas and the separated digestate meet in a counter-current flow in a filter bed. The biogas is mixed with 4% to 6% air before entry into the filter bed. The filter media offer the required surface area for scrubbing, as well as for the attachment of the desulphurizing microorganisms. Microorganisms in the biofilm convert the absorbed H2S into elemental sulphur by metabolic activity. Oxygen is the key parameter that controls the level of oxidation.

The capital costs for biological treatment of biogas are moderate and operational costs are low. This technology is widely available worldwide. However, it may be noted that the biological system is capable to remove even very high amounts of hydrogen sulphide from the biogas but its adaptability to fluctuating hydrogen sulphide contents is not yet proven.

Recycling and Waste-to-Energy Prospects in Saudi Arabia

recycling-Saudi-ArabiaThe Kingdom of Saudi Arabia produces around 15 million tons of municipal solid waste (MSW) each year with average daily rate of 1.4 kg per person. With the current growing population (3.4% yearly rate), urbanization (1.5% yearly rate) and economic development (3.5% yearly GDP rate), the generation rate of MSW will become double (30 million tons per year) by 2033. The major ingredients of Saudi Arabian garbage are food waste (40-51 %), paper (12-28 %), cardboard (7 %), plastics (5-17 %), glass (3-5 %), wood (2-8 %), textile (2-6 %), metals (2-8 %) etc. depending on the population density and urban activities of that area.

In Saudi Arabia, MSW is collected and sent to landfills or dumpsites after partial segregation and recycling. The major portion of collected waste is ends up in landfills untreated. The landfill requirement is very high, about 28 million m3 per year. The problems of leachate, waste sludge, and methane and odor emissions are occurring in the landfills and its surrounding areas due to mostly non-sanitary or un-engineered landfills. However, in many cities the plans of new sanitary landfills are in place, or even they are being built by municipalities with capturing facilities of methane and leachate.

Recycling Prospects in Saudi Arabia

The recycling of metals and cardboard is the main waste recycling practice in Saudi Arabia, which covers 10-15% of the total waste. This recycling practice is mostly carried out by informal sector. The waste pickers or waste scavengers take the recyclables from the waste bins and containers throughout the cities. The waste recycling rate often becomes high (upto 30% of total waste) by waste scavengers in some areas of same cities. The recycling is further carried out at some landfill sites, which covers upto 40% of total waste by the involvement of formal and informal sectors.

The recycled products are glass bottles, aluminum cans, steel cans, plastic bottles, paper, cardboard, waste tire, etc. depending on the area, available facilities and involved stakeholders. It is estimated that 45 thousand TJ of energy can be saved by recycling only glass and metals from MSW stream. This estimation is based on the energy conservation concept, which means xyz amount of energy would be used to produce the same amount of recyclable material.

Waste-to-Energy Potential in Saudi Arabia

The possibilities of converting municipal wastes to renewable energy are plentiful. The choice of conversion technology depends on the type and quantity of waste (waste characterization), capital and operational cost, labor skill requirements, end-uses of products, geographical location and infrastructure. Several waste to energy technologies such as pyrolysis, anaerobic digestion (AD), trans-esterification, fermentation, gasification, incineration, etc. have been developed. WTE provides the cost-effective and eco-friendly solutions to both energy demand and MSW disposal problems.

As per conservative estimates, electricity potential of 3 TWh per year can be generated, if all of the KSA food waste is utilized in biogas plants. Similarly, 1 and 1.6 TWh per year electricity can be generated if all the plastics and other mixed waste (i.e. paper, cardboard, wood, textile, leather, etc.) of KSA are processed in the pyrolysis, and refuse derived fuel (RDF) technologies respectively.

Conclusion

Waste management issues in Saudi Arabia are not only related to water, but also to land, air and the marine resources. The sustainable integrated solid waste management (SWM) is still at the infancy level. There have been many studies in identifying the waste related environmental issues in KSA. The current SWM activities of KSA require a sustainable and integrated approach with implementation of waste segregation at source, waste recycling, WTE and value-added product (VAP) recovery. By 2032, Saudi government is aiming to generate about half of its energy requirements (about 72 GW) from renewable sources such as solar, nuclear, wind, geothermal and waste-to-energy systems.