TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Components: The digester comprises basin adapted to receive sludge; a cover secured to the basin and having an under surface that is adapted to contact the sludge during operation; and a level control weir located above the cover under surface and adapted to allow overflow of sludge from the basin so as to maintain sludge in continuous contact with the cover under surface during normal operation. The digester includes a spray nozzle located in tower. The spray nozzle is used during operation to urge foam within tower over the weir. The tower has a lower horizontal cross-sectional area smaller than a surface area associated with cover under surface. The under surface area is greater than or equal to10 times greater than tower area. The digester includes an overflow collection trough located in said tower. The weir forms an upper edge of trough. The trough is flow connected to a sludge discharge pipe. The weir is a first weir. The weir includes a second weir located lower than said first weir and being joined by a flushing valve to a discharge system. The discharge pipe is a first discharge pipe. The discharge pipe includes a second discharge pipe flow connected through the valve to a floor of said trough downstream of second weir. The cover is rigid and fixedly secured to basin. The cover is sized and shaped to cover all of basin except in the region of tower. The cover is sealably joined to basin around its periphery. The digester includes an overflow weir located at a sludge operating level so as to maintain a constant level of sludge in the digester. The cover includes a central tower that rises above an under surface of a cover of the basin. The weir is located in the tower. The fluid spray nozzle is located in the region and is sized, shaped and positioned to operably spray fluid at foam in the tower so as to direct flow of foam within the region.Preferred Methods: The method includes providing spray nozzles in the tower; and utilizing the nozzles to spray fluid on the foam so as to direct the foam to discharge from the digester. The method comprises placing an overflow for sludge in the digester above an under surface of the cover such that sludge continuously contacts the cover under surface during normal operation.