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by Mantotta K.D. Abeyapala

Pollution of the environment and the damage causing to human and animal health due to unregulated waste treatment methods has become a major problem.

Each city produces tons and tons of waste daily. Industry and comers, resident population and their gardens, they all contribute to creation of waste. It could be produced as liquid or solid waste of which biggest problem today is the collecting, treating and disposing of solid waste. Solid waste may consist of.

* Industrial rejects some of which some may even be hazardous material

* Empty containers, which could be plastic, metal, clay, glass, timber or any other natural or synthetic material.

* Materials like paper, plastic sheets and clothes

* Discarded household furniture and items like fridges, television and other electrical or mechanical items.

* Kitchen waste and food waste

* Garden waste like grass cuttings, pruned trees, fallen leaves and some time uprooted shrubs or trees.

* Waste from construction sites and demolition activities.

Solid waste consist of many types of material some of which may need different approach in collecting, treating and disposing. Collecting, treating and disposing of waste in an appropriate manner to prevent harm to human health and pollution of the environment is the responsibility of the Government.

The government empowers local authorities to carry out this work within their own areas of authority. There are regulating authorities appointed by the governments to oversee that waste Management work is being carried out according to the accepted standards and practices.

However managing waste appears to get low priority in the decision-making process of many local authorities. It suffers from low investment leading to difficulties of recruiting suitable staff or give adequate training to the operatives.

The ineffective waste management practices leads to people creating waste dumps on roadside which are not collected for days or waste been used as landfill material on unregulated landfill sites, without taking any practical measures to mitigate the ill effect of decomposing waste.

Waste dumps whether it is on roadside or on unregulated landfill site affects the lives of the people who live close to such places. It affects them in many ways, including that it may.

* Spoil the beauty of the place

* Cause health hazards

* Affect the quality of life

* Pollute the environment

* Lower the property value.

Therefore each waste management authority should have a well-considered plan of action in the collecting, treating and disposing of waste.

Some authorities especially larger and densely populated ones may have difficulties in finding suitable land for to be used as landfill sites. It is not uncommon for several authorities getting together to form a larger waste management authority to overcome such difficulties and also to share the resources.

Collecting, treating and disposing waste is an expensive process. If we are to minimize the expenditure a thorough study on all aspects of the process should be undertaken by the authorities concerned and in this process all options should be considered. These options include.

Reduce

The first option is to look for the ways and means of reducing the creation of waste. Taking steps to minimize waste creation will result in the efficient use of material and reduce the operating cost.

Re-use

The next option is to look for the ways and means of maximizing the re-use of waste products with minimum processing. This could be done by direct use of the waste for the same purpose previously used or using for a different purpose. (Eg. multiple packaging, soft drink bottles)

Recover value

Recovering value from waste may need sorting, separating and additional processing if such material is to be turned into a different product. Some time value recovery may take place over number of years after initial disposal of the waste. (Electricity from Biogas, recycling of paper, compost).

Selecting or separating any material from the waste is needed only if there is a market for direct sale of such items or there is a recycling plant to use them. Authorities depend on private investors to provide such recycling facilities.

For an investor to start a recycling facility he needs an assured market for his product, a guaranteed supply of raw materials for a reasonable price and assurance that the goal posts will not be moved with the change of the political party in power, which means a long-term contract with the local authority. A demand that many authorities find difficult to satisfy.

Collection

Re-use and recover value options should be given due consideration when reviewing the collection strategy. It may highlight areas where savings could be made to make the process as a whole more economical.

There are many factors that should be taken into consideration when designing a collection strategy. They may include the type of waste, type of the road network and accessibility collection points, method of disposal and distance to the disposal point.

Depending on the road network and the distance to the disposal site, it may become necessary establish transfer stations at various point to carry out an efficient collecting system.

There are various types of vehicles used in collecting waste. They broadly fall into three hands

* Vehicles with no mechanized compression

* Vehicles with intermittent compression

* Vehicles with continuous compression

Generally the degree of the compression decides the amount of the waste carried by the vehicle. Most waste management authorities use a combination of all types.

The type of the transport system used dictates the vehicle combination. The single vehicle method is suitable only for short hauls. It causes collection delays.

The relay system makes use of two or all three types of vehicles. It is an efficient way of using the collecting gangs, and the vehicles. In this system small vehicles collect waste is reloaded to larger vehicles to be taken to the final destination.

The material suitable to re-use or recycle has to be separated at some point in the collecting and disposing process. Where separating should take place, largely depends on the method of collecting.

The single vehicle method will allow separating at the point of collection or at the disposal point whereas the relay system will have three or more points depending on the number of transfer stations.

Industrial waste is treated separately from the household waste at the collecting stage and it is not uncommon to have separate fill site for industrial waste where biodegradable waste is not accepted.

Most authorities now limit the collection of Industrial waste; large household items and garden waste as and when requested on payment of a fee to large and bulky items or there is a lot of it. Instead they run collection points located at suitable places around their area of authority and encourage the public to bring the waste to collecting point themselves. Large containers or skips are kept for the purpose and the authorities remove the containers free when full.

Separating waste at source is the most popular method. This practice is easy to introduce to work places like factories where the production is limited to few items in large quantities of the same, but it is more difficult when comes to household waste.

The common practice is to provide separate bags for each item like paper, empty bottles etc. and get the residents collect their wastes separately in the bags provided by the authorities. These bags will be collected separately or on separate days.

Some authorities tend to arrange collection points at places convenient to the general public to bring and dump such waste at their convenience, by placing separate containers for each of the items and remove the containers when full.

Most authorities collect household waste from the premises on a regular cycle. May be once a week or twice a week on a set day. Some issue standard plastic bin bags to every house and the residents are expected to keep their refuse bags outside the house or at a collection point on the collecting day.

Such bags are made of material strong enough to hold the content without wall tearing and large enough to hold a content that can be carried by the bin man from collecting point to the transporting vehicle.

Where access for refuse collecting are difficult a bank of bins or skips are kept at an accessible point and the residents are expected to dump their waste at this point for collection.

Disposal

Land filling, composting and incinerating are the methods commonly used to dispose waste of which land filling is the most widely used.

Land filling

Land fills are sites where waste materials are deposited on or in to land for the final disposal. This process if not properly carried out has the potential to cause damage to human health and to cause environmental pollution during the operational stage and as well as years after the site has been closed.

The common problems encountered during the operational stage are

* The odour

* Increased fly population

* Loose material getting blown away by wind

* Increase in the population of scavenging animals like rodents and birds

* The surface water run-off polluting the nearby water causes.

* Leachate contaminating the under ground water system

The nature of the chemical composition of landfill site varies; therefore a very wide range of contamination is possible even many years after the closing the site.

Although the site has been closed the site remains active for about 20 years or more and some hazardous material will be there forever. Because of the likelihood of these contaminations, developments of fill site and its surrounding areas can be hazardous and the end use of landfill sites needs careful consideration.

The hazards that are likely to encounter in and around closed fill sites may include

* Much of the waste present at landfill sites is combustible, for example household waste, paper, plastic and rubber. When combustible matter is present below ground, a fire may start and propagate below the surface. Since the supply of oxygen at that depth is limited subterranean fires propagate slowly by smoldering and is difficult to put out. The flames may appear if they break through to the surface.

* Chemical attack on building material and services. Sulphate and acid attack on concrete is the most common problem but some contaminants may encourage metal corrosion and deterioration of plastic.

* Emission of flammable, toxic, asphyxiant or corrosive gases, due to organic mater deposited in landfills getting decomposed by the action of micro-organisms. Initially it is due to the activity of aerobic (oxygen requiring) microorganisms and the product includes carbon dioxide Co2 and water.

As decomposition proceeds the release of these gases prevent the ingress of air and decrease the supply of oxygen available to microorganisms. Eventually aerobic activity ceases and organisms capable of surviving under anaerobic (oxygen deficient) conditions become dominant.

During the early anaerobic phase Co2 production increases along with some hydrogen (H2). Methane (CH4), which is inflammable, usually dominates the gases generated under anaerobic conditions but the presence of Co2 and residual nitrogen (N2) may modify the properties of the mixture. Such mixture is known as landfill gas.

The properties and the behaviour of landfill gas depend on the ratio of Co2 and H2. Methane itself forms explosive mixture with, air in the range of 5%-15%, but these properties are modified by the presence of Co2.

As the proportion of Co2 increases the flammability of the mixture decreases. if the ratio of Co2, and CH4 is greater than 3.5 the mixture will not be flammable if mixed with air in any proportion. This ratio is known as "limiting safe mixture (LSM).

However such mixtures are seriously deficient in oxygen and may present an asphyxiation hazard. There may also be a noticeable odour due to the presence of trace compound such as organic sulphur compounds in the gas. Waste with high Sulphate content may produce H2S on anaerobic decomposition.

* Flammable or explosive concentrations can also be produced, when landfill gas is mixed with air and infiltration of these gases in to near by buildings and confined spaces is a possibility. The infiltration of gas also has an adverse effect on plant growth since it depletes the oxygen in the root zone.

* The surface water run off from the landfill may pollute the watercourses and leachate may pollute the ground water.

* Due to gradual settlement of the site under its own weight and due to corroded metal containers in the fill collapsing may create site settlement problems.

* Presence of substances that can affect human health such as lead, cadmium, nickel, arsenic and cyanide which may be harmful even at relative concentration. These may have a direct effect on humans or the possibility that some may get to the food chain cannot be ruled out.

Therefore the practice of using any convenient site as a landfill site is not an acceptable way of disposing waste. Most countries now have rules and regulations against such practices.

Realizations that there is value to be recovered from waste, the improvements in technology, and increased rules and regulations to protect the environment has made most local authorities to adopt the concept of controlled land filling on engineered fill sites. These sites adopt internationally accepted standards and practices of waste disposal.

To minimize ill effects of the operating phase it is the recommended practice that waste is deposited in layers. The thickness of each layer is kept below two meters immediately after consolidation and limits the working area to an adequate size to avoid vehicle congestion.

The each layer of waste deposited and compacted is then covered with a layer of gravelly soil. The thickness of which is to be not more than 250 mm. It is known as the "Primary Cover" and is placed over the days work before winding up for the day.

The Primary Cover is placed in order to

* Reduce the amount of loose material getting blown off by wind. Providing peripheral chain link fence will further reduce this menace.

* Reduce the unpleasant smell by retarding the diffusion of gas with the atmosphere.

* Discourage the scavenging birds and rodents

* Reduce the fly population by prevent flies laying eggs in the waste pile and also prevent the new born flies from the lava in the already deposited waste coming out.

To get the maximum benefit from the biogas produced and to stop leachate and gas migrating to pollute the adjoining lands, modern fill site are designed according to the accepted engineering principles.

On new engineered site, a mineral liner (clay) that has a permeability of 1*10-9m/sec-1 throughout a minimum thickness of one meter, or a synthetic liner with similar properties is installed throughout the base and the sides before any waste is deposited. On completing the fill to designed levels, the site will be closed with an impermeable liner similar to one used on the base and the sides placed over the top of the fill.

The ends of the top liner are then fused with the sideliners to form a watertight joint to entomb the entire fill. To complete the closure minimum of 300 mm thick soil cover is placed over the top liner, which is known as the protective soil cover. The placing of the top liner and the soil cover is called capping the site.

The depths of the protective cover vary according to proposed end use of the fill site. If trees are to be planted the soil layer need to be increased to support the root system of the proposed variety of trees. The minimum thickness may be sufficient for grass and wild plants.

To collect the biogas a network of inter-connected perforated pipes are buried within the fill, which has venting points at pre-determined places. These could be used to release gases to the atmosphere or to extract gases for commercial purposes.

A system of drainage pipes buried under the fill will collect leachate to strategically placed manholes through which the excess leachate could be pumped out.

Once the site is capped top impermeable liner will stop rainwater percolating through the fill, reducing the amount of leachate produced. The gas production will also increase since it prevents the gas escaping freely.

Production of leachate and biogas in appreciable quantities takes place from a year of placing up to about 20 years. The use of biogas an energy source is well known.

Methane rich biogas, comprising about 55% Methane and 45% carbon dioxide with small amount of other trace gases is used for generating electricity and purposes other than electricity including furnaces; boilers and kiln firing.

It can also be used for firing brick kilns as a cheap energy source. Advancements in technology have made it possible to use biogas efficiently in generating electricity.

The gas within the landfill site is warm and saturated with water vapour, as it leaves the landfill site it cools and water condenses out. The gas is dewatered, cleaned and compressed before it enters an engine for the purpose of generating electricity. Stand by flares also has to be in place, so that excess gas can be burnt off for environmental control and as a means of controlling the gas when the plant is closed for maintenance.

The target set by the Landfill Gas Association for generation of electricity in UK alone by the year 2005 is 800mw and there are over 150 sites in operation and the number is increasing at a rate of 10% a year.

Composting is a biological process that converts solid organic waste into a product (compose) that can be used in agriculture, horticulture and other markets. This sustainable waste management system is being encouraged as an alternative to land filling for bio-degradable waste. However it should be noted that only about 20% of all waste collected would be of organic origin and suitable for composting.

All composting methods require the waste separation at some point to remove the bio-degradable waste content from the rest. The separated bio-degradable matter is then pulverised to get into a size suitable to encourage rapid composting.

Composting systems range from simple to very sophisticated. The two methods available are called the "Open System and the Closed System".

Open systems are mostly used when dealing with large quantities and where land sufficient to facilitate such operations can be found. The Open system has two methods one is called the windrow method and the other called the static pile composting method. They are easy to operate simple systems and have a low investment cost and low operational cost. However both methods can lead to problems of leachate and odour but could be minimised by good operational practices.

The closed systems are carried out in purpose made containers. The system is subdivided according to:-

* Whether the composting material moves vertically or horizontally.

* Whether forced are been applied.

* Whether it is been carried out continuously or in batch form.

What system to use depends on the amount and the type of waste to be composed but these systems has the advantage of been able to customise the containers to suit amount of waste that needs to be handle.

Almost all modern incinerating plants belong the direct incinerating process although there are a few old separation incineration plants around the world. Direct incinerating process aim is to maximise volume reduction by burning refuse as it is received.

Managing waste is not an attractive business. Waste is dirty, smelly and unhygienic to handle, therefore even the authorities whose duty is to manage waste give it a low place in their list of priorities. Management of waste if not given its due respect will lead to environmental pollution and will cause serious harm to human health.

Today the waste management technology has improved and there are plenty of commercial benefits to be had from a properly managed system.

What is required is more investment on waste so that better methods can be introduced along with better-trained operatives to manage the systems. This in turn will result in having a cleaner environment for all.

(The writer is a Chartered Civil Engineer, a member of the Institution of Civil Engineers (London) and a Fellow of the Institution of Engineers (Sri Lanka).

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