Extracting Value from Sewage Sludge

Increasing world population combined with desire to live within an urban environment is putting strain on the earth’s resources. In a recent article (Rus, 2011), six resources are key with respect to sustaining human life on earth. These comprise three energy sources (coal, oil and gas), water, phosphorous and rare earth elements. Of these, only rare earth elements cannot be directly recovered from wastewater and sludge. Ironically rare earth elements are not rare as their name suggests, however their significance is due to their location as China currently controls 97.5% of global market share. Although wastewater and sludge are resource‐rich, wastewater treatment works were designed to destroy rather than recover value. Water within wastewater is routinely returned to waterways and not used, whilst nutrients such as nitrogen and phosphorous are destroyed using chemical and biological nutrient removal techniques, leaving only between 15 to 20 % of the nutrients being recovered within the biosolids exiting the works assuming it is used in agriculture as a fertilizer product. Regarding energy recovery, this is achieved on a wastewater treatment works using anaerobic digestion. However, as the original driver of having anaerobic digestion was one of sanitation and not energy recovery, the quantity of energy recovered from sludge in this way is modest. Only 40 – 50% of energy is recovered under standard conditions, and this rises to approximately 60% with best practice techniques involving pre‐treatment. Subsequently, there are opportunities to improve recovery of value from today’s wastewater treatment works, and success of these opportunities will be dependent on the level of urgency to recovery value and economic viability.

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