The importance of properly designed and constructed landfills

Regardless of where we live, work, or play, we generate waste. To ensure our waste doesn’t harm public health or the environment, modern landfills are technically sophisticated and highly regulated. These landfills are commonly referred to as ‘municipal solid waste landfills’ to distinguish them from the open dumps of the past.

By Stan Jewaskiewitz*

Unlike old dumps, modern landfills have sophisticated protective liners, leachate collection systems, groundwater monitoring, gas collection equipment, and environmental reporting requirements.

Waste disposal is still, in many cases, carried out by dumping. Such dumpsites are often characterised by:

• indiscriminately dumped heaps of uncovered wastes, which are sometimes burning

• pools of standing, polluted water

• rat and fly infestations

• feral and domesticated animals roaming freely

• where poverty is rife, families of scavengers picking through the wastes. Such dumpsites are usually (but not always) outside the urban areas of towns, located in areas not suitable for any other purpose.

The reasons that such conditions are accepted are varied and numerous, but may be summed up as:

• ignorance (of the health risks associated with dumping of wastes), or acceptance of the status quo, due to: – lack of financial resources to do anything better – lack of political will to protect and improve public health and the environment. A transition from open dumping to sanitary landfilling would include intermediate stages that might be described as:

• designated dumping (within a designated site, but with no control of operations) • controlled tipping (in a supervised site, with wastes disposed of in an organised manner, in layers and covered periodically)

• engineered landfilling (where the impact of wastes on the environment has been assessed and engineering measures taken to limit, but not necessarily eliminate such impacts).

Progression from one such ‘standard’ of operation to the next requires significant increases in technical competence at the local/site level and in financial resources to sustain it.

There are opportunities for incremental improvements at every stage of development of a waste disposal site: from its initial siting, through design, construction and operation, to site closure and aftercare.

In the past, little regard was given to where landfills were located and how they were operated. Today’s modern landfills are built in locations that protect human health and the environment, as well as the structural integrity of the landfill.

The greatest potential to safeguard the local environment from adverse impacts from a new landfill, without incurring the excessive cost of protective measures, is in selecting an appropriate location. Locating a landfill in the right hydrogeological setting can often avoid the significant expense of importing materials to create a basal barrier system to protect groundwater.

Many other site selection criteria also have a bearing on the cost of site development and operation, and on the cost to the waste management system as a whole, including:

• distance from areas of waste generation

• access from good public roads

• likelihood of local (e.g. on-site) availability of cover material

• proximity to municipal sewage treatment works (if leachate is to be collected). In the search for a new landfill site, the conversion of the existing dumpsite may also be considered.

In an ideal world, a number of potential sites should be identified, and a systematic process of selection/rejection applied to yield one, two or possibly three preferred sites, on which full technical, financial and environmental assessments are then carried out.

Unfortunately, in the real world, more than one potential site may be difficult to find, formal assessments are seldom undertaken, and the replacement site may be chosen on the basis of availability alone.

In under 35 years, landfills have changed from little more than holes in the ground to highly engineered, state-of-the-art containment systems requiring significant capital expenditure.

Typically, older landfills were designed by excavating a hole or trench, filling the excavation with waste, and covering the waste with soil. In most instances, the waste was placed directly on the underlying soils without a barrier or containment layer (liner) that prevented leachate (water percolating through the waste and picking up contaminants) from moving out of the landfill and contaminating groundwater.

Dumped garbage was often openly burned to save space for future waste disposal, creating air pollution and health hazards. When the waste reached a predetermined height, a final cover of soil was placed on top and sometimes vegetation was planted.

In many cases, the vegetation failed to grow or died because of methane gas (a natural by-product of waste degradation) escaping through the final cover. Also, the landfill gas could move offsite into buildings and homes, potentially creating health and explosion risks.

In contrast, modern landfills are specifically designed to protect human health and the environment by controlling water and air emissions. Figure 1 provides a typical cross section of a modern landfill.

Liquid containment within a modern landfill results from a combination of the liner and the leachate collection system performing complementary functions to prevent groundwater contamination. Liners prevent leachate and gas migration out of the landfill while directing liquids to the leachate collection system.

Liner systems are typically constructed with layers of low-permeability, natural materials (compacted clay) and/or geosynthetic materials (e.g. high-density polyethylene). The leachate collection system removes the liquid contained by the liner. A typical leachate collection system may consist of (from bottom to top) a perforated leachate collection pipe placed in a drainage to dispute in principle: certainly, few politicians would wish to actively support a scheme that might be considered less than 100% free of risk.

Waste is placed directly above the leachate collection system in layers. Delivered waste is placed on the working face that is maintained as small as possible to control odours and vectors. Heavy steel-wheeled compactors move the waste into the working face to reduce the waste’s volume. At the end of each day, the waste is covered with a layer of soil or an alternative daily cover (foam, tarps, incinerator ash, compost) to control vectors, odours, fires, and wind-blown litter.

Once the landfill has reached its permitted height, the landfill is closed and engineered to prevent water infiltration by installing a low-permeability cap similar to the liner system. The final cap can be comprised of a compacted clay and/or a synthetic material. A granular drainage layer is placed on top of the low-permeability barrier layer to divert water off the top of the landfill. A protective soil cover is placed on top of the filter blanket and topsoil is placed as the final layer to support vegetation.

In short, the sophisticated engineered systems in a modern landfill ensure the protection of human health and the environment by containing leachate that can contaminate groundwater, preventing the infiltration of precipitation that generates leachate after closure of the landfill, and collecting landfill gas, which can be used as an energy source or destroyed.

Perhaps the most symbolic feature of modern sanitary landfills is the quality-assured, fully engineered basal liner, complete with integral leachate collection and removal systems. Their provision is required by environmental regulations in most developed nations and, though seldom enforced, in an increasing number of developing countries. The justification for protection of the groundwater environment in such a way is difficult

Having carried out a rigorous site selection and design process, it is imperative to ensure that proper construction procedures for the landfill and its various components are carried out in accordance with an approved CQA plan. This is the best ‘guarantee’ the owner of the landfill site can have that the landfill will not pose a threat to the environment.

Despite legislation and policies on waste management, which emphasise the reduction and reuse of wastes with the concomitant need to divert waste away from landfill, very little progress has been made in this regard in recent years – all of this while the available airspace in existing landfills is dwindling and, in some cases, non-existent.

As in the past, landfills will therefore continue to play an important role, in the foreseeable future, in our country’s waste management systems until such time that alternative waste treatment technologies are put in place.

However, with proper design and construction processes, gone are the past problems associated with older landfills, such as groundwater and air contamination, acceptance of hazardous waste, and inappropriate locations in sensitive areas.

Modern landfills, in contrast, are highly engineered containment systems that are designed and operated to minimise the impacts of municipal solid waste disposal on human health and the environment.

*Stan Jewaskiewitz, Pr Eng, is the technical director at Envitech Solutions.