For several decades it has been an accepted practice in the United States for the final product from wastewater treatment plants, sludge, to be applied to large tracts of land. This is usually done by spraying, as the sludge is still liquid or semi-solid. This practice depends on the availability of many acres of land, and the area usually has to be fenced from public intrusion due to the possibility of the remaining presence of pathogens in the biosolids.
Increasingly, land application of wastewater treatment products is being questioned. Although it was a better practice than the landfilling of sludge, and much better than discharge into bodies of water, sludge is still an incomplete, secondary treatment product causing environmental concerns. Currently, several technologies are being developed to convert biosolids into biofertilizers. This is a tertiary treatment option.
Biofertilizers are different from chemical fertilizers in that they depend upon the presence of living organisms for their effectiveness. Types of life forms in biofertilizers include fungi, bacteria and algae. Each of these groups forms specific relationships with plants, particularly in the rhizosphere, the root zone, which enhance the ability of plants to grow. While a chemical fertilizer contains nitrogen, phosphorus and potassium, a biofertilizer may contain only low levels of these nutrients, but the primary advantage is that the biofertilizer helps plants use these nutrients. Typically, these organisms enter into the plant roots and form symbiotic relationships with plant cells.
The concept of producing biofertilizers from wastewater is not new. Several brands of compost made from municipal waste have been on the market for years. Heat processing has been used to kill pathogens, and reduce the sludge to a solid condition where it can be handled, which is known as dewatering. Lower water content also reduces the weight, making transportation more economical. However, dewatering is an expensive process.
In recent years, other methods of converting municipal wastewater sludge to biofertilizer have been explored. One such method uses vermiculture. This is the addition of worms to a solid sludge. Over a period of 21 days the biological processes of the worms increased nitrogen, phosphorus, and potassium levels in the sludge. Their biomass also increased.
Specific wastewater streams, those from particular industries rather than general municipal waste water, lend themselves to transformation.
Another trial has used Azolla (a small water fern) to treat waste from pig farming. In this case the growth of the Azolla plant also increased the nitrogen and phosphorus content of the waste product, and made it suitable for use as a biofertilizer.
The palm oil industry creates huge quantities of waste. Transformation of husks into biocharcoal creates a composting material that provides a soil amendment product to trap nutrients near plant roots, making them readily available for use.
Land application of wastewater products after secondary treatment has been the industry standard. However, advances in treatment options are providing possible ways to reduce the volume of waste sent for land application and to transform waste products into valuable biofertilizer commodities.