8.4.2 Deep Row Entrenchment

Deep row entrenchment is a controlled disposal method for sludge in which deep trenches are excavated, filled with faecal sludge/septage, and subsequently covered with soil. To this extent, the method can be considered for the safe sludge burial. However, depending on the site, deep row entrenchment is to incorporate sludge into the soil to improve soil fertility and structure while providing nutrients to crops or plants. The sludge placed in these trenches undergoes decomposition and assimilation by soil microbes, contributing organic matter and nutrients beneficial for plant growth.

What are the requirements regarding sludge?

A dry matter content of 20% to 35% is recommended. This range helps balance the moisture content to encourage decomposition and, at the same time, minimize leachate formation to be disposed of. Local environmental regulations should be consulted to ensure compliance.

What are the requirements regarding the site?

Deep row entrenchment requires soil with good permeability and high infiltration capacity. If the soil is too clayey or has a rocky stratum, the infiltration will be low, so more trenches and a larger area may be required. The method is unsuitable for regions with heavy rainfalls, high groundwater tables, and regularly occurring flooding. In such wet environments, the risk of leaching from the trenches, which can cause pollution of groundwater and surface water bodies, would increase.

Parameter	Recommendations
Distances	> 50 m to surface water bodies. > 200 m to human habitation
Groundwater table	Bottom of the trench should be 5 – 10 m above the highest water table level.
Soil	Sandy and loamy soil

What are essential design considerations?

In addition to distances to water sources, implementing a deep row entrenchment requires the consideration of trench dimensions and method of filling. As the figure shows, the trenches are usually up to 2 m deep. The trench is filled with sludge up to 0.3 m from the top of the surface and then backfilled with excavated soil.

Generally, the following dimensions can be considered when designing a deep-row entrenchment. When setting up a layout, it is recommended to divide the site into four parts to be prepared quarterly, considering a year-wise cycle.

Parameter	Description
Depth of trench	1 – 2 m (distance bottom trench to the highest water table to be kept in mind)
Bottom width of trench	0.3 – 1.5 m
Top width of trench	1.25 – 5.5 m
Length of trench	Up to 10 m (generally not specified, but depending on site and area available)
Space between two consecutive trenches	2 – 3.5 m (depends on soil and vehicle movement amongst other things; in case of loose soil or larger vehicle, space of around 5 m is recommended)
Side slope	0.50:1 or 1:1
Depth of sand layer at the bottom of the trench (if deemed necessary)	0.3 m
Trench lining (if deemed necessary)	Layer of plastic or agri-film

Depending on the proximity of water bodies or groundwater tables, there are different variations of the possible design:

• Deep trench with sand barrier: If the groundwater depth is close to the recommended limit, but no surface water sources are nearby, a thick layer of sand (about 0.3 m) should be provided at the bottom of the trench to reduce leaching of solids and pathogens.
• Deep trench with sand barrier and plastic/agri-film cover: If both groundwater and surface water sources are almost equal to the recommended distances, potential contamination is advised to be prevented by applying the sand layer at the bottom of the trench as well as plastic/agri-film cover for all the four sides of the trench. This will prevent seepage through the sides and divert leaching through the bottom sand layer.

What are the space requirements?

When narrow trenches (1 – 3 m wide) are used, sludge total solids content can be lower than 30% since the trench side walls will support it. This disposal requires large areas but allows operational simplicity and is recommended for small sludge volumes. It may accommodate 450 to 2,100 tons of sludge cakes (dry basis) per hectare, including areas between trenches.

Large trenches (3 – 15 m wide) allow trucks access to the disposal ditches to unload sludge. They require solids concentrations higher than 40% to support vehicle traffic. It may accommodate 1,200 to 5,500 tons per hectare.

How can a DRE site promote land application and reuse of sludge?

If sludge is dewatered, stabilized and pathogen-free due to the preceding treatment, the treated sludge is compatible with the specific soil, and the treated sludge is applied in a sustainable practice, then DRE can promote land application and reuse of sludge.

Based on these conditions, a filled-up entrenchment can be utilized and planted. Planting trees creates a nutrient sink, reducing the movement of potentially harmful nutrients from the trench. This may be an opportunity to produce timber for building, fencing or fuel. Plant species with a high water demand can assist in absorbing leachate produced by sludge, reducing the risk of groundwater contamination. Further, plant species that have high nutrient consumption rates are recommended. This includes, amongst others, eucalyptus, acacia, poplar, etc.

The following picture illustrates the transformation from an unused field to a thriving forest – an outcome achieved through the combination of deep-row entrenchment and agroforestry.

For further information, please click on the Materials tab at the top of the page.