Biosolids Utilization on Agricultural Land - Phosphorus

Biosolids are nutrient rich processed organic materials derived from municipal wastewater treatment processes. Biosolids usually contain mineral and organic nitrogen, phosphorus, potash, organic matter and micronutrients such as zinc, magnesium, and copper. The utilization of biosolids as part of a farm nutrient management package can reduce the demand for commercial fertilizers, improve soil fertility, and enhance soil structure, moisture retention and permeability. Biosolids are ideal for crops such as corn, soybeans, cereals and forages.

One of the major crop nutrients in municipal biosolids is phosphorus (P). "Phosphorus is vital to plant growth and is found in every living plant cell. It is involved in several key plant functions, including energy transfer, photosynthesis, transformation of sugars and starches, nutrient movement within the plant and transfer of genetic characteristics from one generation to the next." (Better Crops Vol. 83, 1999 No.1).

P Availability

Municipal biosolids contain an average of about 3% elemental phosphorus (dry weight basis). That is 30 kg of elemental P, which converts to 69 kg of P₂O₅ per metric tonne (dry weight). Some of this P is in the liquid fraction of the biosolids and is highly available. Some of it will be bound in the organic matter that will break down over time and potentially release the phosphorus for plant uptake. Some of the P will be tightly bound in aluminium, iron and other chemical compounds that are very slow to react and may never release the P.

The quantity of P in each fraction depends very much on the method of treatment used in the sewage treatment plant (STP). STP's that utilize alum or ferric compounds for phosphorus removal from the wastewater stream will contain higher percentages of Al and Fe bound-P than STP's that use biological P removal technologies. Further processing of the municipal biosolids by heat treatments, such as pelletization, further decreases the quantity of available P. Research by Dr. H.A. Elliott & G.A. O'Connor found that the percentage of water extractable phosphorus (the most readily available) was significantly lower when compared to livestock manure and inorganic fertilizer P. (J.E.Q. 31:681-689,2002) But in later work they found that the quantity of plant available P (the fraction available to the crop in the year of application) from alum or ferric treated biosolids was very similar to that of livestock manure, at 40% of the total P. The P availability from biologically treated biosolids was higher, while the availability of P from pelletized biosolids was significantly lower. This research demonstrates that as a source of crop available phosphorus, municipal sewage biosolids are approximately equivalent to most livestock manures.

Application Rate

The significant difference between manure and biosolds in the field is the quantity of material that is generally applied in a single application. Most liquid livestock manure application rates range from 4000 to 6000 gallons per acre. Depending on the available nitrogen content and the application method used, municipal biosolids applications may go as high as 15,000 gallons per acre. This biosolids application rate can apply up to 3 times the amount of P in a manure application. That is enough for multiple years of crop production. This is one of the reasons why municipal biosolids applications are limited to a set application quantity, either a solids or available nitrogen loading, over the 5-year duration of a Certificate of Approval.

Municipal biosolids are a good source of organic matter and crop nutrients, particularly phosphorus. But as with any crop input, Best Management Practices need to be followed to ensure efficient crop utilization. Nutrient Management Planning must be a key component in any municipal biosolids utilization program and both the quantity of P applied and the relative availability of the nutrients in the biosolids needs to be considered.

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