Nutrient reduction capacity of a novel bioswale system for the reduction of nutrients from farmyard dirty water

Abstract

The protection of clean water is of paramount importance to both humans and the aquatic environment. The aquatic environment in Ireland is subjected to impacts from many different human activities and pressures. Pollution from agriculture comes in the form of point sources from farmyards, containing animals and diffuse pollution, where nutrients and pesticides are washed off land and into the rivers through rainfall. Farmyard dirty water (FYDW) is created when rainwater gets mixed with agricultural waste streams. FYDW can be a mixture of urine, animal feces, organic matter, milk parlor waste and inorganic pollutants that contain nutrients. These waste steams cause serious pollution of ground and surface water bodies. FYDW may also contain potentially pathogenic micro-organisms, pesticides and antibiotics. These constituents can have detrimental effects on the aquatic environment as well as affecting human health. At present, there are two ways of dealing with FYDW, land spreading which is regulated in the European Union (EU) and open to misuse, and Integrated Constructed Wetland (ICW) which can require large areas of land. In 2015 a feasibility study was carried out on campus at the Institute of Technology, Carlow. This study examined the ability of a small scale bioswale containing an aquatic plant layers of soil, wood bark and a submerged layer of pea gravel. The study conducted over 3 months found good reductions in chemical oxygen demand (COD), nitrate, and ammonium but not reactive phosphate. The objective of this study is to develop a novel bioswale for the treatment of FYDW. The system is relatively small in comparison to ICWs, the novel bioswales could provide a more cost-effective way of reducing pollutants present in FYDW. The objective of this study was to evaluate materials that could be used in a bioswale to remove phosphorus (P), three experiments were carried out under controlled conditions. The first experiment was a screening of several materials and examination of effective flow rates for phosphorus (P) removal. The material should act as a choke layer to prevent smaller particles from the soil above from descending into the drainage layer into the bottom of the system. Dolomite removed 87.8-99.6% of P from solution. Shale removed 98.1% and soil removed 98.5%. The second experiment used Langmuir sorption isotherms too examine the effectiveness of dolomite, shale and soil in the removal of P from solution. The results of the second experiment gave theoretical values for the sorption of P in mg per kg of material with soil, shale and dolomite. The experimental values for the sorption isotherms were 0.545 mg g-1 for dolomite, 0.143 mg g-1 for shale and 0.587 mg g-1 for soil. This enabled the building of 6 small scale bioswales, 3 for each type of bioswale, the dolomite and shales bioswales. The final experiment seen the construction of PVC pipes with 5 layers of the materials, soil, willow bark, shale or dolomite and limestone as a drainage stone. Three pipes had a layer of dolomite and three had a layer of shale. The findings from these experiments showed that dolomite and shale both had high levels of removal of reactive phosphate removal from solution. The percentage reductions were 97.65% for shale columns and 95.23% for dolomite columns. Under negative redox conditions there was a significant difference between the two types of bioswales. The shale bioswales had a better total phosphate removal than the dolomite bioswales under the same conditions. The overall conclusion was to use soil as this was required for the plant and had good P removal properties, shale and dolomite in any future bioswale. The final experiment also examined the removal of ammonium and nitrate from solution. The systems removal rates of ammonium were above 94.0%, with the dolomite bioswale systems having a higher percentage removal rate than the shale systems. For both types of bioswales it was noted that there was a steady decline in the removal of both nitrate and ammonium. The Langmuir isotherm experiments conducted on the materials for the removal of P and the values obtained for the removal of ammonia and P from solution, should now allow for the development of a large scale bioswale construction and the monitoring of nutrient removal from such over an extended period of time.