Tank with membrane cover and draft tube mixer

A wastewater treatment system including an anaerobic digester, the digester having a tank with a rim, comprising:

• a membrane gas cover on top of the tank and covering the tank, connected to and sealed to the tank rim to be inflated and contain gas therein,
• an interruption in the gas cover at least at one location around the periphery of the tank, with a platform secured to the tank rim at the interruption, the platform including a substantially horizontal surface, the gas cover at said interruption having edges on each side of the interruption connected in substantially sealed relationship to the platform, and
• a draft tube mixer secured to the platform and extending down through an opening in said horizontal surface, the draft tube mixer including a draft tube secured to the platform and to the tank, fixedly secured therein, and a mixing mechanism having a shaft and an impeller removably fitted in the draft tube, with the impeller and shaft extending down into the draft tube and a drive mechanism connected to the shaft and secured at the top of the draft tube, the draft tube having openings above and below the impeller, for drawing in sludge from within the tank and expelling the sludge in a desired direction, thus circulating and mixing the sludge in the tank.