Agricultural benefits of biosolids

The recycling of biosolids to agricultural land is currently considered the best practicable environmental option by the UK government.

Biosolid products contain good quantities of most major plant nutrients, organic matter, and valuable amounts of micro nutrients. The use of the nutrients contained in organic materials such as biosolids is a sustainable way of reducing the amount manufactured fertiliser products used by farmers.

Nitrogen

Depending on the crop type, time of year and product type, biosolids can supply around 10-15% of the nitrogen needs of a crop.

The nitrogen supplied is mainly in the organic form and consequently it is released slowly as the soil warms, and is therefore present over the crops entire growth cycle.

Phosphate

A typical biosolids application will replace crop offtake and help maintain or build up soil phosphorus (P) to target levels.

By maintaining these target levels using biosolids it will help to obtain optimal yields by reducing the risk of P deficiency.

Sulphur

Sulphur has become an important crop nutrient as deposition from the atmosphere has decreased.

When applied at standard application rates the levels of sulphur supplied in biosolids will supply enough to match typical cereal and oilseed rape crop requirements.

Magnesium

Biosolids applications contain significant amounts of magnesium which can help provide a maintenance dressing for soil and prevent deficiency symptoms (sometimes seen on light sandy or acidic soils).

Micro nutrients

Micro nutrients are normally required by plants in relatively small amounts depending on the crop and soil type, if not addressed properly they can be a cause of crop deficiencies.

Biosolids contain a wide range of micro nutrients including boron, copper, zinc, iron, sodium, manganese, aluminium and molybdenum.

Organic matter

Biosolid products contain a large amount of organic matter. Organic matter is a vital component of good quality agricultural soils, and helps to provide a good growing medium for crops.

Repeat applications of biosolids can help maintain and build up soil organic matter levels, especially in intensive arable farming where there is often no alternative organic matter source. The key benefits of organic matter include improved:

  • soil structure
  • rooting
  • drainage
  • soil workability
  • water retention
  • soil biology
  • reduction in soil erosion.

Product analysis

A typical biosolid digested cake (25% dry matter) product applied at a rate of 20 t/ha can provide the following.

Nutrient Nutrient content (kg/t) Applied total nutrient (kg/ha) Potential availability year one (%) Available nutrient year one (kg/ha)
Total nitrogen 11 220 15 33
Total phosphate (P2O5) 11 220 50* 110
Total potash (K2O) 0.6 12 90* 11
Total sulphur (SO3) 8.2 164 10-20** 16-33
Total magnesium (MgO) 1.6 32 - -

* Depending on soil P and K indices and crop being grown
** Assume 10-20% available following autumn applications and 20% following spring applications

 

Quality assurance with HACCP

Southern Water incorporates Hazard Analysis Critical Control Point (HACCP) quality assurance systems within its treatment processes. These systems ensure that the required bacterial standards for biosolid products are guaranteed.

The HACCP system was developed in the food industry to ensure that the required standards at each process stage are continually met. In the UK the same HACCP approach has been adopted by the Water Industry for its treatment of raw sludges to treated biosolids.

Under this system limits are set for factors which are critical to maintaining end product quality. These factors are defined as Critical Control Points (CCPs) under the HACCP system. By operating the treatment process within the CCP limits, the end product quality is assured.

 
 
 
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