Miss Natarajan, it's not easy to find a concrete answer because there are many factors involved in the removal efficiency of BOD, COD in anaerobic tanks. This depends very much on the selected technology, operational parameters, also sewage organic matter content etc. The removal efficiency could be low (less than 60%) but very high as well (>90-95%). There are waste water treatment norms that impose strict values for these parameters, so as the producers of such facilities care to fulfill the norms. For example, in Europe the norms stipulate a COD reduction to at least 75-100 mg/l (from 600-900 mg/l that is generally in domestic sewage), this means a reduction efficiency of ca. 88-89%. For BOD, the reduction must be from ca. 300 mg/l to 25 mg/l, this means efficiency of 91.7%. Most of certified technologies can achieve even higher COD, BOD removal efficiency. Also, many lab researches show a COD and BOD reduction efficiency >90% for the anaerobically treated sewage.

May be this research will help.