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Treating variable strength landfill leachate and wastewater

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Gas wells at a landfill site. Extracting methane from landfills produces condensate, a wastewater with higher ammonia concentrations than leachate.

By Susan Rennie

With environmental integrity in mind, development of the Seneca Landfill site in Pennsylvania started long before it was built in 1965. A team of professionals conducted several comprehensive tests and surveys prior to the start of construction, including geographical impact (geological and hydrogeological testing, wildlife inventory and monitoring) as well as traffic, historical and demographic research to ensure minimal impact on both the community and the land around it.

In areas around landfills, concern regarding water contamination compels older and modern sites alike to be proactive in their effort to keep groundwater clean.

Seneca Landfill’s wastewater treatment plant has the capacity to process up to 422,000 litres per day. It uses a multi-stage treatment system to process the landfill’s contaminated water so it can be released safely into a nearby creek without any negative impacts to it, or downstream receiving waterbodies.

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The sources of this water are condensate and deliquification water (both produced during gas pumping), as well as leachate from sources like rainwater that mixes with the waste from the landfill and becomes contaminated. Modifications and continued studies on environmental impacts showed the amount of ammonia in the wastewater from these sources was proving to be too much for the existing system.

The need for change

Following a study by the U.S. Public Interest Research Group in 2000, all sites that discharge into the creek became subject to stricter ammonia permits on all wastewater being released. In its un-ionized form, ammonia is toxic to aquatic life in receiving waters as it causes increases in both temperature and pH levels. Additionally, as a nutrient, ammonia can promote excessive algae growth, resulting in the loss of dissolved oxygen in rivers, lakes and oceans.

Landfill leachate is a common waste type characterized by extreme BOD and ammonia spikes. While the average ammonia level in the influent is 1,400 mg/L, Seneca Landfill has reported days where the influent has been as high as 5,000 mg/L. To comply with their permit, the landfill needed to meet an on-site ammonia effluent limit of 4.7 mg/L. Being unable to consistently meet that target using their own system, they had been forced to incur additional costs for off-site treatment.

Part of the cause of these high ammonia levels was the desire of the landfill owners to extract methane gas produced by the site. Traditionally, landfills use flares to burn off both excess methane and CO₂. Instead, Seneca Landfill wished to capture the methane and convert it into a clean form of natural gas for powering the landfill itself and other customers. However, the methane extraction process produces condensate, which is a wastewater with an even higher concentration of ammonia than the original landfill leachate. This rendered the existing wastewater treatment system insufficient.

The original system at Seneca Landfill consisted of three bio-towers filled with outdated trickling filter media. At the time, only two were in operation. All of this contributed to the system being unable to reduce effluent ammonia to required levels. A decision was made to convert the existing bio-towers to aerobic BioPortz™ MBBR reactors from Nexom, a Canadian wastewater company, which would eventually run in series. The bio-towers would be updated one at a time with new BioPortz media.

How it works

Wastewater from the landfill is first collected in two equalization tanks, which allows operators to control the flow rate to the system. From the equalization tanks, water is pumped through the metals removal system. Metals removal is accomplished by the addition of sodium hydroxide (caustic) as well as ferric chloride. Next, the wastewater is sent to a sludge separator for primary sludge removal.

In many cases, the concentration of the various compounds in the leachate changes from day to day, depending on the amount and composition of waste being collected by the landfill, as well as precipitation, temperature, and a variety of other factors. This provides a challenge for the operators of landfill leachate wastewater treatment plants. There are still occasions when there is too much ammonia coming into the system to be treated.

The highest noted flow rate into the reactors during the tested time period was about 106 L/min, but landfill operators found the system consistently treated wastewater to below effluent standards at a flow rate of 76 L/min.

To counteract this issue, the system allows landfill operators to redirect effluent back to equalization tanks for further treatment when needed. As well, to help facilitate ammonia removal and maintain consistency for the biomass in the reactors, the wastewater is heated to roughly 27°C. After heating, the water is sent to the BioPortz reactors.

Fundamental to MBBR treatment is that biomass is easiest to retain when it is fixed to a surface. When the intent is to remove BOD₅, nitrify ammonia, or even denitrify nitrates, growing bacteria on a surface minimizes the washout that occurs in a suspended-growth environment. BioPortz media is optimized for surface area in order to encourage attached bacteria growth, ensuring that the entire media surface area is available to biomass. Created out of high-density polyethylene (HDPE) with carbon black, the dimensions of each piece of media are carefully engineered to balance size and shape to enable both coarse media retention screening and effective media motion.

Seneca Landfill uses three BioPortz reactor tanks in series. Each tank holds 72 m3 of water and contains 48 m3 of media. In total, there are some 83,612 m2 of available attached-growth surface area.

BioPortz reactor tanks
Seneca Landfill now uses three BioPortz reactor tanks in series to treat wastewater on site.

Wastewater enters the fully aerated tank and encounters the biofilm-coated media. Coarse effluent screens are also installed to prevent the patented BioPortz media from leaving the tank. Depending on the desired result, multiple tanks in series can be used to meet low effluent requirements and/or address several different contaminants and nutrients.

Updating the towers

The retrofit of the first bio-tower to a BioPortz MBBR reactor was completed within six months. Updating the towers one at a time ensured minimal disruption and downtime for the landfill. It also provided an opportunity to measure effluent characteristics at three different intermediate flow configurations along the way, in addition to the final reactor configuration.

The first configuration consisted of wastewater flowing in series through one of the non-retrofitted bio-towers filled with old media, followed by the newly upgraded BioPortz MBBR reactor. At this configuration, average ammonia removal efficiency was measured to be 88.9%, with a standard deviation of 17.0%.

The second bio-tower was completed two months later. This second configuration saw wastewater flow through the BioPortz MBBR reactors in parallel. At this configuration, average ammonia removal efficiency was 95.5%, with a standard deviation of 7.3%.

The third configuration consisted of two BioPortz MBBR reactors in series. At this stage, ammonia removal rose slightly to 98.5%.

The project was completed when the third BioPortz reactor came online six months after that. The final configuration consisted of wastewater from the landfill flowing through the three upgraded BioPortz MBBR reactors in series. Initially, there wasn’t a significant improvement once the third reactor was put online, but further testing revealed greater benefits once the biomass was fully established. Loading rates were decreased to the third reactor, which allowed the biomass to adapt appropriately and produce the more expected result.

Upgraded system performance

The average influent flow rate the Seneca Landfill towers experienced in the first 10 months of operation was 69 L/min. The new system, designed to achieve effluent ammonia concentrations of 4.7 mg/L, has been consistently able to achieve this limit or less, therefore enabling them to safely discharge their wastewater. By April of the year following completion, the creek was no longer listed in the top 10 most polluted waterways in the U.S.

In addition, after the updated BioPortz MBBR reactors were put in place, the landfill was able to achieve its objective of converting landfill gases, specifically methane, into enough green energy to power not only one third of the landfill’s daily operations, but also to heat over 27,000 homes. It also supplies compressed natural gas for fueling CNG vehicles, including their own fleet.

References available on request. Susan Rennie is with Nexom. This article appears in ES&E Magazine’s October 2019 issue.

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