One of the primary mechanisms for treatment of wastewater within a lagoon system is solids settling. Settled solids contain dirt, sand, and other debris as well as biological floc, inorganic compounds, heavy metals, and nutrients. Over time settled solids, referred to at this point as sludge, accumulate within the lagoon. Eventually the depth of sludge will reach a point where it needs to be removed and disposed in order that the lagoon system can continue to operate the way it was designed. Elevated sludge levels reduce hydraulic detention time, reduces treatment performance and can lead to increased odor.
Sludge measurement and sampling
The rate at which sludge depth increases within a lagoon system is dependent upon influent wastewater characteristics as well as the effectiveness of the lagoon system. Typically, sludge from a lagoon system requires removal every 20-30 years. To monitor the rate that sludge is building up in your lagoon, sludge depth measurements should be taken every 5 years. Once you’ve determined that the sludge in your lagoon needs to be removed, a composite sample will need to be obtained to determine the nutrient and metals content of the in-place sludge in order to best evaluate how it may be disposed.
Processing & Disposal
Disposal of sludge is typically through direct burial at a landfill, composting (typically at a landfill), or through land application. Disposal of sludge at a landfill is required if the metal contents within the sludge exceed the ceiling concentration limits set forth by EPA Rule 503. Landfill disposal can be costly, as the quantity of sludge to be disposed can be significant and tipping fees at the landfill can add up. However, this type of disposal may prove cost effective even when metal concentrations are acceptable; if a suitable site for land application isn’t available. It’s also worth asking your local landfill if they have a composting program where they can utilize the sludge. Sludge is a great source of organic material, necessary for any composting program; and tipping fees can be greatly reduced if the sludge can be composted. Landfill disposal also requires drying or dewatering sludge to pass a paint filter test. In our experience this requires dewatering to a minimum solids content of 15% by weight.
Land application means to take advantage of the nutrient content and soil conditioning properties inherent of the sludge by applying it directly to arable land. If the composite sludge sample meets the requirements of EPA Rule 503 metal contents, this is typically the most cost effective method of sludge disposal. Dewatered sludge is applied at agronomic rates to typically alfalfa or grass hay crop land, based upon the nutrient content of the sludge and nutrient uptake of the crop. This prevents leaching of the nutrients below the crops root zone which may affect ground water quality. Though a permit is not always required, depending on the size of your community, several requirements, including pathogen and vector attraction reduction, of the EPA Rule 503 must be met.
Removal of in-place sludge presents several hurdles. Bypassing the treatment lagoon for a significant amount of time is often not feasible, and access for physical removal without significant damage to liner material is unlikely. Often a floating dredge pump is employed to remove sludge without needing to remove the lagoon from service and prevents damage to the lagoon liner material.
Typical consolidated sludge within a lagoon has a solids content of 1-4%. This means for every pound of solid material, there are 96 to 99 pounds of liquid. Regardless of the disposal method chosen, it is financially beneficial to remove as much of the water from the sludge as possible.
Several methods are available to reduce the amount of liquid and increase the solids content to reduce the volume of sludge that needs to be disposed. The correct method for your treatment system is dependent on a number of factors including land availability and schedule.
Temporary lined drying beds rely exclusively on evaporation to remove water from sludge and require significant space and time, as well as ideal weather conditions to achieve a dried sludge. The addition of a granular filter material and perforated subdrains to collect filtrate can greatly reduce the time and space required to dewater.
Several geotextile suppliers have begun manufacturing large fabric bags with connections for dredge pump outlet piping. The fabric is permeable, to contain sludge solids while allowing liquid separation through the matrix of the fabric. The bags can be installed within constructed drying beds with pumps for filtrate return flow, or if space allows, they can be installed directly on the lagoon embankment. Once the desired solids content is reach, the bags are cut open and the dried sludge is removed with a skid steer and haul trucks. The addition of a properly designed polymer feed system can speed up the process and help yield an even higher content sludge cake.
If land for construction of drying beds is unavailable at the treatment site, mechanical dewatering equipment such as a belt filter press can be rented. Although this dewatering technique can be costly, a uniform dry solids cake can be produced quickly. Training on the rented equipment is typical, or a trained operator can be provided by the rental company.
If sufficient land for disposal is available close to the lagoon site, direct injection of liquid sludge from the dredge pumps can be used. This technique utilizes a system of hoses connected to a tractor that injects the liquid sludge below the ground surface meeting the requirements of vector reduction for land application specified in EPA Rule 503.
Great West Engineering has performed alternative analysis for disposal of sewage sludge for several communities. Regardless of the dewatering technique employed or disposal method used, removal of sludge from your system is critical to maintain the effectiveness of the treatment process. From composite sludge sampling to site specific design of dewatering methodologies, Great West Engineering can help navigate the technical and regulatory framework to determine the most efficient technique for your treatment system.
For more information on Wastewater Lagoon Sludge Disposal please contact Steve Lipetzky, PE ([email protected]).
Authored by: Steve Lipetzky, PE