Rice mills release a lot of wastewater. Paddy has to be soaked in water for 6-8 hours and dried for de-husking. Water is also used otherwise in the boiler. Parboiling paddy also uses water. All this liquid waste goes down the drain. Since the wastewater has ammoniacal nitrogen and phosphates, it poses a problem for the environment.
There have been various attempts to remediate the effluent. But, so far, they have not been up scaled for commercial applicability.
So J Umamaheswari and S Shanthakumar from the Vellore Institute of Technology decided to tackle the problem using Scenedesmus obliquus. The microalga has been found suitable for remediating other kinds of wastewater and for nutrient removal from rice mill effluent. How efficient is the microalga in removing ammoniacal nitrogen and phosphate? What is its growth potential in raw paddy-soaked wastewater? What is the biomass productivity? What is the pattern of the biomass yield coefficient when scaled up from polybags to photobioreactor and raceway ponds?
The researchers collected wastewater locally, from a rice mill in the Tiruvannamalai district. They found that paddy-soaked wastewater was slightly acidic, with high biological and chemical oxygen demand.
For testing, they used a Scenedesmus obliquus strain from a rice mill industrial site. They cultured it under three different cultivation systems: polybags of two litre capacity, photobioreactors with fluorescent lamps that kept a dark-light cycle of 12 hours each and raceway ponds of seventy-five litre capacity. The polybags were manually shaken, the photobioreactors fitted with pumps and the raceway ponds with paddle wheels to provide aeration.
After seven days the team measured the parameters under consideration.
They found that maximum removal of ammonical nitrogen and phosphates was achieved in three days in the photobioreactors and raceway ponds and in two days culture for poly bags. However, polybag cultures removed less phosphates and ammoniacal nitrogen from the wastewater. Greater than 90% removal efficiencies were achieved for biological oxygen demand and chemical oxygen demand in photobioreactors and raceway ponds. More than 80% of chlorides and 60% of total suspended solids and turbidity was also noticed.
The biomass yield depended on the pH. If the pH is kept between 8 and 9, the yield can go up to about 340 milligrams per litre per day with photobioreactors. It is slightly less in raceway ponds and even less in polybags where the optimum pH turned out to be less than 7.
There was nearly 250% increase in biomass in photobioreactors. The researchers correlated the differences in the yield from photobioreactors and raceway ponds in terms of the illuminated surface area.
More than 10 percent of the biomass harvested from both photobioreactors and raceway ponds was composed of lipids – which is a measure of its value for biofuel production.
Much higher lipid yield can be achieved by optimising the algal culture parameters in laboratory conditions. But the researchers have shown that it is possible to scale up the technology to reduce the environmental pollution posed by rice mills and, in the process, produce an energy crop.
Nourishing news that may sound sweet to rice mill owners.
J. Environ. Manage, 243: 435-443 (2019);
Central University of Kerala
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