Multiple tank high solids anaerobic digester

Abstract

A multi-stage anaerobic digester is designed to treat a high solids, stackable feedstock. The system may also receive a pumpable feedstock such as a slurry or sludge. In a first stage, the digestate circulates in one direction around a raceway such that the digestate may pass a feed inlet multiple times before leaving the first tank. An optional side stream loop withdraws fibrous material from near the top of the raceway and return digestate with chopped fibers, preferably lower and further along the raceway. An outlet from the raceway located near, but upstream of, the feed inlet discharges partially digested substrate to a second stage, which is operated as a stirred tank reactor. The two stages may be provided in a single tank with an internal wall separating a ring shaped outer portion from a cylindrical inner portion. The digester may be operated in a thermophilic temperature range.

What is claimed is: 
     
         1 . An anaerobic digester comprising,
 a) a covered cylindrical tank; and   b) an interior wall dividing the cylindrical tank into an outer portion and a cylindrical inner portion.   
     
     
         2 . The digester of  claim 1  further comprising a passageway between the outer portion and the inner portion. 
     
     
         3 . The digester of  claim 1  wherein the outer portion is ring shaped. 
     
     
         4 . The digester of  claim 1  further comprising a set of mixers spaced around the outer portion and oriented to direct digestate in one direction around the outer portion. 
     
     
         5 . The digester of  claim 1  further comprising an inlet for adding a substrate to the outer portion. 
     
     
         6 . The digester of  claim 5  wherein a first of the set of mixers located closest to the inlet in the one direction has a mixing blade that is located higher than the mixing blades of other mixers in the set of mixers. 
     
     
         7 . The digester of  claim 6  wherein the height of the mixing blades decreases from the height of the mixing blades of the first mixer along the one direction. 
     
     
         8 . The digester of  claim 1  further comprising a side stream in which digestate is withdrawn from a location within one meter of the free surface of the digestate in the outer portion and the withdrawn digestate is returned to the outer portion at a lower elevation than the withdrawal location. 
     
     
         9 . The digester of  claim 8  wherein the withdrawn digestate is returned to the outer portion at a position displaced in the one direction away from the withdrawal location. 
     
     
         10 . The digester of  claim 8  wherein the side stream comprises a chopper pump. 
     
     
         11 . The digester of  claim 8  wherein an inlet to the side stream comprises a crust breaker. 
     
     
         12 . The digester of  claim 1  wherein the outer portion has at least 40% of the volume of the digester. 
     
     
         13 . The digester of  claim 1  further comprising an inlet adapted to introduce a stackable substrate into the outer portion and further comprising a second inlet adapted to introduce a pumpable substrate into the outer portion. 
     
     
         14 . The digester of  claim 2  wherein the passageway comprises a riser such that digestate flows upwards while moving from the outer portion to the cylindrical inner portion. 
     
     
         15 . The digester of  claim 1  further comprising an outlet for removing substrate from the outer portion. 
     
     
         16 . The digester of  claim 1  further comprising an inlet for adding a substrate to the outer portion and an outlet for removing substrate from the outer portion. 
     
     
         17 . The digester of  claim 16  wherein the outlet is downstream from the inlet. 
     
     
         18 . The digester of  claim 1  wherein the inner portion is an undivided inner portion.