Tauranga City Council - WWTP

The plant operates two anaerobic digesters each with a volume of 1260m3. Primary sludge from the bottom of the primary tanks is pumped to the digesters where it is broken down by a different process. This is called an anaerobic process because air is kept out of the digesters. The bacteria in the digester operate differently from those in an aerobic environment. Through the break down of protein, sugars, starches, fats, carbohydrates and organic matter they produce organic acids such as acetic acid, the constituent of vinegar. Then a very special type of bacteria takes over. These bacteria are called methonic bacteria and they convert acids to methane gas and carbon dioxide.

Approximately 2,300m3 of gas is produced daily from the anaerobic sludge digestion and is utilised to run a generator, producing up to 2500 kW of power, thus helping to supplement the plant's power demand. As a result of the generator burning gas, heat is generated; this is used to maintain the digester temperature between 35 and 37oC.