India is a leading producer of milk now, with an annual production touching 100 million tonnes. Raw milk is processed to produce consumer milk, butter, cheese, yogurt, condensed milk, milk powder and ice cream. Buttermilk, whey, and their derivatives are typical by-products.
However, the dairy industry produces a large amount of wastewater – more than twice the amount of milk processed. This wastewater becomes a pollutant. It contains nutrients for bacterial growth making it an environmental threat.
But what if it can be used to generate electricity employing microbial fuel cells? Biswanath Bhunia and team from NIT, Agartala decided to investigate.
Their experimental fuel cell, an acrylic chamber with a working capacity of 200 millilitres, had a carbon anode and a platinum/ carbon cathode with a membrane as separator. The anode and cathode were connected by a stainless steel wire.
For the initial experiments, the team mixed two grams of milk powder in a litre of distilled water to simulate dairy water. Then, they added the nutrients and trace elements necessary for microbial growth and to maintain alkalinity.
Escheria coli was chosen as the microorganism for the fuel cells. The researchers injected E. coli into the anode chamber through an inlet port and maintained anaerobic conditions. The cathode chamber was kept open.
Voltage overshoot is a key drawback of microbial fuel cells. So the team used a fed-batch reactor to ensure reasonably stable voltage generation.
They then collected real dairy wastewater from a milk producer in Agartala. The chemical oxygen demand of this wastewater was more than 8000 milligrams per litre. The researchers diluted it to get wastewater with chemical oxygen demand in the range of 500 to 2000 milligrams per litre to check which concentration gives the most stable output. They found that a chemical oxygen demand of 1000 milligrams per litre gave the best power output. So the same concentration was maintained in the batch experiments.
The total duration of the experiment was 6 days. Open-circuit voltage increased during the first 67 hours and reached a maximum of 0.661 volts after one and a half days – a little less than half the capacity of a dry battery cell, which typically gives 1.5 volts.
As the process progressed, the concentration of milk powder decreased after three days, and the pH shifted to the acidic range. The voltage decreased to 0.550 volts.
So, in the first feed, the team added 50 millilitres of higher concentration simulated dairy water of pH 9 to maintain the concentration for maximum E. coli growth. The microbes grew exponentially and current generation increased from 10 to 67 hours. To stabilize continuous open-circuit voltage, the team fed the fuel cell at regular intervals.
After the first feed, the open-circuit voltage increased within 3 hours and reached a maximum of 664 millivolts after 80 hours. After five days, there was a second feeding with real dairy wastewater. And a maximum open circuit voltage of more than 650 millivolts was observed. The microbial fuel cell achieved a high current density of 3.60 amperes per square metre.
The researchers look forward to scaling up these microbial fuel cells for industrial scale application. Treating wastewater is expensive. But not if it can be used to generate electricity to recover the costs.
Fuel, 266: 117073 (2020); DOI: 10.1016/j.fuel.2020.117073
Khushbu K Birawat, Global Academy of Technology