Annex the local landfill to your wastewater treatment plant and defer expenditure on treatment process upgrades - and keep nutrients out of the local waterway.
Municipal wastewater treatment plants typically receive waterborne waste from the communities they serve. Within the treatment plant these wastes are separated in sedimentation or filtration processes into a liquids waste stream and a solids waste stream. These waste streams are generally treated in aerobic process units (liquid stream) and in anaerobic process units (solids stream) to reduce contaminants to levels that are considered safe for the wastes to be released to the environment.
The treated liquids stream (effluent) is most commonly disposed of to a water environment (wetland, river, estuary or in a coastal zone) although there are some smaller wastewater schemes in New Zealand, located in areas of lower than average rainfall and with soils of high permeability, where the effluent is disposed of by irrigation.
The “solids” stream is treated in the wastewater treatment plant in a dilute form (in wastewater comprising 0.5 to 2.5% solids). Water is then removed using presses, centrifuges or drying beds to raise the percentage of solids to approximately 20% where the dewatered/dried sludge (biosolids) can be disposed of to a landfill. If the biosolids are relatively free of metals, organics and pathogens they may be disposed of, as a soil additive, to public areas such as parks and reserves.
Modern landfills are designed with impermeable base liners and very low permeable finished capping layers so that moisture (predominantly rainfall) is excluded. The refuse is dry entombed to minimise the generation and escape of leachate (moisture that is formed through the decomposition of organic wastes present in household wastes and/or rainfall that percolates through the refuse and becomes contaminated). The leachate is removed through a network of collection drains on the base of the landfill. It is then treated in a dedicated wastewater treatment plant and released to the environment. Alternatively, the leachate is conveyed to the local wastewater treatment plant for co-treatment with the municipal wastewater.
Slowly, the regulatory approach of dry entombment is changing.At a few modern landfills, leachate is being recirculated through the refuse to increase the rate of refuse stabilisation. This shortens the post closure, aftercare period from approximately 30 years to possibly as short as 10 years. Additional benefits include partial treatment of the leachate and an increase in the density of the compacted refuse which provides the opportunity to dispose of a greater volume of waste in the same sized landfill/bioreactor.
Interestingly, in the current state of this changing regulatory paradigm, there is insufficient leachate produced at some modern landfills, especially during the drier summer period, to fully achieve the conversion from dry tomb landfill to bioreactor. The opportunity therefore exists in some situations to import more liquid waste to create a bioreactor.
Back at the municipal wastewater treatment plant environment, regulators are applying increased pressure on the owners of these municipal assets to continue to improve levels of treatment of the wastewater, before its release to the environment. The regulatory attention is now focussed on the increased percentage removal of the contaminants, nitrogen and phosphorus. Both can create nuisance conditions in waterways with the proliferation of aquatic plants during summer and possibly the longer term effect of eutrophication in downstream lakes or large estuaries.
For owners of municipal wastewater treatment plants that receive high strength, low volume wastes such as industrial discharges, landfill leachate and/or that generate such wastes in the plant from sludge dewatering, (the latter two both high in nitrogen and phosphorus), consideration should be given to using the local modern landfill as a disposal pathway for these wastes. Collaboration with the landfill owner is clearly required but the process is far easier if the local authority is the owner of both assets.
Harrison Grierson is currently working with a council on the renewal of the resource consent for its wastewater treatment plant. The plant receives on average 20 m3 per day of landfill leachate through the summer months and generates approximately 15 m3 per day of centrate from the process of dewatering the biosolids. The combination of both wastes adds 17Kg daily of nitrogen and 6 Kg of phosphorus in the discharge to the local waterway which could be redirected to the council’s modern landfill for treatment and disposal. This integrated approach to waste disposal and environmental protection provides benefits at the sites of both the council’s assets as well as deferring expenditure on upgrades to process equipment at the wastewater treatment plant site.
This thought leadership article by Colin Cranfield, a Technical Director at Harrison Grierson, is intended to provide you with insights and relevant information about waste disposal and environmental protection. Our thought leadership articles on topical and specialist issues are designed to present the key points in an easy to digest and interesting manner.
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