The curves in Figure 1 do tell us something about the nature of the substrates tested, however. Cellulose fibers are pure polymeric sugars and highly digestible. Their high energy content results in a higher specific methane yield than either peanut hulls or sewage sludge. The digested sewage sludge produced very little methane compared to peanut hulls. Peanut hulls are a raw, intact natural product. Sludge is solids settled from treated sewage – sewage that could have once been peanut hulls before the microbes in a sewage treatment plant removed much of their easily digested OM. Generally speaking, the further along in a treatment process you go, the lower the COD and BMP of the treated material will be.