Oxygen boosts efficiency in leachate treatment
For landfill operators, leachate treatment is a challenging aspect of wastewater management. Some are finding that using pure oxygen as part of an enhanced aeration process can significantly improve efficiencyA landfill site in Wales has recently installed a system which uses pure oxygen to pre-treat leachate so it can be released safely as wastewater into the sewer. The Bryn Posteg landfill site, which opened in 1982, is owned and run by Potters Waste Management and is located close to Llanidloes in mid Wales.
Licensed by the Environment Agency for the disposal and treatment of household waste, the site has always been a full containment site, which uses natural impermeability and engineered systems to prevent leachate from being discharged into the water environment. Located in a remote area, the site has a 1,200m3 leachate lagoon, and the landfill operator had been exploring ways to upgrade its wastewater treatment systems.
The lagoon was coping well with a steady increase in capacity. But its exposed location meant that the water temperature had been making the surface aeration process less effective, particularly during winter months. So, it was sometimes difficult to achieve the specified discharge limits for some leachate contaminants, and an enhanced pre-treatment system was needed.
Air Products provided a solution using high purity oxygen, as Philip Whittaker, the company's business development lead for water systems, explains. "Traditional aeration systems can be severely affected when the temperature dips below 15oC as nitrification of the water is inhibited, and there is no ammonia removal. At this particular site, the leachate has a high ammonia content, with a maximum strength of around 2,000mg/l.
"Any significant drop in temperature meant that the discharge consent limit for ammonia content, set at just 150mg/l, became extremely difficult to achieve."
Traditional aeration systems work by reducing the oxygen demand of the wastewater in the lagoon or treatment tank by dissolving as much oxygen from the air as possible. The aeration process encourages the growth of natural microbes or bacteria that cleanse the water and remove organic and other contaminants.
By using high-purity oxygen, in place of atmospheric air, the new technology works in the same way. But the oxygen is dissolved more quickly. By increasing the rate at which the oxygen is dissolved, and reducing the amount of gas passing through the wastewater, there is reduced heat loss from the process. The wastewater is also less to prone to foaming. The capacity and efficiency of the process are thus enhanced.
Working in partnership with Potters Waste Management, Air Products specified the replacement of two surface aerators with a single Oxy-Dep VSA system, which is capable of delivering 875kg a day of oxygen. Despite the site's remote location, the modular system was installed quickly and easily, without the need to drain down the lagoon.
David Williams, technical manager at Potters Waste Management, says: "Since installing the Oxy-Dep VSA system, we have been able to keep the ammonia content of the leachate well within the specified discharge consent limit. In the year following installation, we were able to treat more leachate while maintaining the same discharge quality. The whole process is more efficient, and the capacity of the process has increased. Since upgrading the system, we are now able to treat an extra 10% to 15% of our leachate per day."
Achieving discharge limits is not the only driver for landfill operators to review their leachate and wastewater treatment systems. Tightening environmental legislation, in particular the UK's implementation of the Landfill Directive and the Water Framework Directive, which has set a deadline for all surface and groundwater to achieve good ecological status by 2015, is also having an impact.
In response, most forward-thinking waste management companies and landfill operators are taking action now to ensure their treatment systems are designed for optimum efficiency.
Philip Whittaker says: "Operators are looking for ways to upgrade their existing systems so that they are capable of exceeding the requirements of tighter environmental legislation now and in the future. In doing so, they want to minimise site disruption, keep overheads down, and if possible increase the capacity of the treatment process too."
The benefits of new oxygen-based aeration systems are:
- low emissions
- high shock load resistance
- high efficiency and treatment rates
- easy installation
- oxygen permits/building regulations may not be required.