8.3 What are the conditions and requirements to safely bury sludge?
Adherence to local and national governing the disposal of sludge is fundamental. These regulations may define the following requirements:
- Required pre-treatment,
- Permissible levels of contaminants,
- Site selection criteria and
- Containment requirements to safeguard public health and the environment.
Most importantly, an uncontrolled disposal site needs to be avoided at all costs. Illegal dumping, which may be an easier and cheaper option, will cause poor environmental and public health hazards with long-lasting effects.
Required Pre-treatment
Before disposal, the sludge should be stabilised and dewatered. Some sources imply that sludge characteristics such as degree of stabilisation or pathogen level are not a primary concern when choosing sludge burial as the final disposal route. However, an official controlled disposal site requires, according to available regulations, sludge stabilisation with a focus on reducing the organic fraction to prevent uncontrolled degrading processes once the sludge is disposed of. Untreated sludge produces gases that can be collected during treatment, minimising emissions after final disposal.
Sludge dewatering has a similar importance in guaranteeing the mechanical stability of the sludge mass. Disposal trenches can be adapted to the solids content to a certain extent but require at least 20%. Conventional dedicated sludge landfills require at least 40% solids content in sludge (preferably 60%).
Permissible Levels of Contaminants
Although household-generated sludge generally has lower heavy metal concentrations compared to industrial and commercial sources, it can still contain increased levels. This is because household products such as cleaning agents, detergents, personal care products, and certain pesticides are often disposed of through toilets. Also, improper disposal of household waste, such as batteries or electronic devices, can cause increased pollutant concentrations. While treatment processes will reduce the sludge volume and stabilize the organic content of the sludge, more steps may be needed to remove heavy metals effectively.
If it is known that sludge is contaminated with heavy metals that exceed national regulations, then sludge burial in the soil is advised to be avoided. It may be disposed of in a landfill that is bound to containment conditions such as a liner and leachate collection system.
Site Selection Criteria
An essential consideration for implementing sludge burial is the availability of suitable land not far from the preceding treatment plant. Besides approval from the environment agency and full compliance with standards, the neighbouring population of a future burial site should be listened to, and their concerns should be considered during the design and implementation phases.
The level of complexity of site selection and design detail to be considered during this phase is a function of the volume of sludge to be disposed of. The essential factors to consider are soil, surface water, groundwater, and site stability.
| Characteristics | Recommendations |
|---|---|
| Soil | Top- and subsoil layers should be assessed in terms of structure, permeability, and cation exchange capacity. A clay content of at least 20% and a pH level above 6.5 is recommended. Permeability coefficient k <10-7 cm/s is recommended. |
| Surface water | If there are any water bodies near the burial site, these should be identified and sampled to determine baseline values. A distance greater than 30 m is recommended: >200 m from sole-source aquifers, strategic aquifers, recharge zones for major aquifers and boreholes |
| Groundwater | Groundwater quality should be assessed to establish baseline values for monitoring. Water table depth is recommended to be greater than 1.5 m (based on the highest occurring water table throughout the year, better >5 m |
| Site stability | Valleys and hillsides with slopes below 20% are recommended to minimize issues related to erosion, runoff and instability. E.g., trenches should be dug parallel to the ground contours, not parallel to the slope direction |
Containment Requirements
The disposal site needs to be fenced to avoid any accidents. This applies to small as well as larger-scale sludge burial sites. Implementing a burial site at least 200 m from habitable properties is generally advised.
If the recommendations regarding soil, surface and groundwater cannot be fulfilled or sludge is known to be contaminated, it is advisable to apply a liner. A liner (e.g., a synthetic geomembrane like high-density polyethene or PVC) functions as a barrier that will prevent the migration of harmful substances or pollutants from the sludge into the soil, groundwater, or nearby surface water bodies. If the sludge is not dewatered before disposal, leachate will form as precipitation filters down through the sludge. If soil and water conditions are not given and sludge is not sufficiently stabilised, lining is advised. Additionally, the leachate should be collected at the bottom. The leachate collection system is a device installed above the liner. The leachate is drawn off and disposed of, effectively eliminating groundwater contamination risk.
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