The most common method of processing biosolids is through carefully monitored composting. This method is very similar to decomposition of organic material in a natural system. A recent study done by the National Pingtung University of Science and Technology investigated the potential benefit of incorporating spent active clay in to biosolid composting. Spent active clay is a by-product of industrial food oil processing. “Unspent” active clay is employed for its ability to attract and affix impurities to itself when added to an oil. Once impurities are attached to the clay it is filtered out and the remaining oil is safe for the kitchen. Worldwide, 900 000 tons of spent active clay are left over from food oil processing every year.
Biosolids are usually composted with the addition of dry organic matter such as straw, rice husks or another readily available plant fiber. This method will temporarily affix heavy metals to some degree but once these compounds decompose fully the heavy metals will again be available for uptake. The aforementioned study investigated the incorporation of spent active clay into biosolid composting in order to affix toxic heavy metals into forms that will not be readily absorbed by plants or water. Unspent active clay has a negative electrical charge which causes its attractive properties. When spent active clay is added to a compost pile, small microbes will break down the residual impurities from its previous applications. Once these impurities are removed, the clay regains its attractive properties. This study has found that the reactivated clay will then attract and affix heavy metals, removing them from the agricultural process more effectively than other composting methods.
Active clay, also known as bentonite, is a naturally occurring group of compounds that are obtained for industrial purification processes through mining. Active clay deposits are a finite resource, making the reuse of active clay especially important.
The effect of incorporating active clay into biosolid composting was tested over a 15 week period. Biosolids, rice husk and spent active clay were mixed at an initial ratio of three parts biosolids to one part each of rice husk and spent active clay. The biosolids were treated with lime to neutralize the acidifying properties of the spent active clay. These constituents were mixed homogenously and left to decompose with occasional turning. Throughout this process the total content of heavy metals, organic carbon, nitrogen, and phosphorus was tested at regular intervals. After fifteen weeks of composting, the resultant product was found to have a desirable ratio of carbon to nitrogen. This ratio is considered the major indicator of compost maturity. As well, the heavy metals present in the final product were found to be in compounds unlikely to be taken up by a plant’s roots or washed into the water supply.
This recent study suggests that the incorporation of spent active clay into large scale composting of biosolids is an important step that can greatly increase the safety of the final products. Further, it has the additional benefit of making maximum use of active clay, a finite resource. Unfortunately it is impossible to destroy heavy metals, however it is possible to affix them in unreactive compounds which are not likely to be inadvertently absorbed into a living body. The application of spent active clay, previously considered a waste material, has been shown to be effective in creating these safer compounds. It is through this type of important research that our uniquely human way of living becomes closer to one that we can continue without significant degradation of nature.
C.P. Ho, S.T. Yuan, S.H. Jien, Z.Y. Hseu. 2010. Elucidating the process of co-composting of biosolids and spent active clay. Bioresource Technology, 101: 8280-8286.
By Dylan Harding