Recovery of phosphate from animal wastewater

An OU graduate student, Nusrat Sharmin, is conducting research regarding the ability of calcium silicate hydrate, synthesized from silica-rich rice husks, to remove phosphorous from ruminal wastewater. The phosphorous can then be used as a fertilizer in place of the typical diammonium phosphate (DAP) which is of vital importance for food production.

The research work of Nusrat Sharmin seeks to recover phosphate from an animal manure waste stream.

Her advisor, Dr. Elizabeth Butler says that "This research is part of a larger project that aims to make phosphorus use more sustainable by recovering it from wastewater using low cost materials made from agricultural wastes such as rice husks.  The project accomplishes several things:

• recovery of phosphate from concentrated animal wastes for use as an alternative to expensive commercial fertilizers;
• recycling and reuse of a limited resource (phosphorus) that is essential for food production; and
• finding a beneficial use for a waste material (rice husks) that can be difficult to treat."

Sharmin’s studies so far have shown promising results with up to 99% removal of phosphorous in low-alkalinity water.  However, the cost of one ton of phosphate recovered from silica-rich rice husks was 6.7-10x more expensive than the standard DAP fertilizer. Therefore, Sharmin is exploring different alternatives to synthesize the calcium silicate hydrate, such as lime, which has an expected cost reduction of 84% from the previous rice husk method. 

Rice husks are the outer hull that protect the interior rice kernel that is the staple food for 3.5 billion people who get 20% of their calories from rice.

If Sharmin’s studies prove to be successful, this could have huge implications for developing countries where farmers often lack the resources to afford adequate fertilizer. Since lime can be extracted from limestone and/or dolomite rather simply, the ability to recycle wastewater and produce fertilizer is both low-cost, environmentally friendly, and would provide small-scale farmers the resource to improve their crop yield. This method would also remove the excess phosphorous from municipal and/or ruminal wastewater, effectively minimizing the occurrence of eutrophication which can be particularly harmful to plant life and aquatic species.

Excess nutrients - nitrogen and phosphorus - that make their way into lake or pond will cause eutrophication, low oxygen levels, and fish die-offs. 

Regarding the application of her work in developing nations, Sharmin says "Disposal of rice residues like husk and straw is a big problem for rice-producing countries as often they are burned in open fields causing smoke and breathable dust containing crystalline silica, that leads to diseases related to lungs and eyes. So utilizing this waste as a water treatment material and generating a high value product (phosphorus fertilizer) would be very beneficial to agriculture industry."

There may be some challenges however. She continues: "One challenge of using rice residues in water treatment is that the potential of arsenic contamination. Arsenic tends to accumulate in rice crops, particularly if the paddy soil and irrigation water (groundwater) is high in arsenic content. It also depends on the nature of rice species. So it is necessary to examine the arsenic content of rice husk/straw samples before applying it in water treatment."