ABSTRACT
The land application of liquid sludge has a venerable history and has been used for centuries to improve crop yields. Sludge was land applied in the US for generations before the advent of cheap, plentiful chemical fertilizers in the 1950s, when the practice became virtually obsolete. The present trend in wastewater treatment toward the land application of sludge is not a new development, but the revival of an old solution. Sludge is applied to agricultural land as a substitute for commercial chemical fertilizers. The primary difference between sludge and chemical fertilizer is that the volatile solids in sludge typically total more than one half of the total solids, making sludge a good source of organic matter. Amending cropland with sewage sludge increases crop yields substantially. The nitrogen and phosphorus contents are only part of the story; each of these elements can be added by applying inorganic fertilizer.
