Date of Award


Document Type

Master Thesis

Degree Name

Master of Engineering


Civil, Structural & Environmental Engineering

First Advisor

Denise Barnett


Phosphorus is an important nutrient for plant and animal life. It is one of the three essential nutrients required for plant growth; Nitrogen, Phosphorus and Potassium. It plays a vital role in the quality and structure of plants. In animals it is essential for the transfer of energy around the body and for the development of the skeleton. Phosphorus is mined from deposits of phosphate rock and is mainly used to produce fertiliser which is applied to agricultural land to improve plant growth. However, phosphorus is a finite resource and, similar to oil, is expected to peak during this century. After this peak, phosphorus will become increasingly scarce and expensive. Unlike oil, phosphorus cannot be replaced by other sources and it is therefore imperative that the peak of phosphorus, followed by its depletion, is offset.

Phosphorus is currently recovered from wastewater streams through wastewater sludge. The sludge is typically treated and applied to land as a replacement to fertiliser. The dairy industry is one industry which carries out land spreading of its wastewater sludge. The sludge from the dairy industry currently offsets less than 2% of phosphorus containing fertiliser (P fertiliser) in Ireland. However, this may increase as consumption of P fertiliser is declining in Ireland.

There are also two issues which may have an impact on dairy wastewater sludge; the abolition of the milk quota system in 2015 and the possible future ban on land spreading of wastewater sludge. The abolition of the milk quota system is expected to cause an increase in milk production in Ireland of 11.6% by 2020. This will cause some increase in the production of dairy wastewater sludge. However, there is a limited landbank to which the sludge may be applied. Therefore land spreading may not be an option. Switzerland has stopped land spreading sewage sludge and now incinerates all of its sewage sludge. Should Ireland decide to ban land spreading of sludge, a new method of dealing with the sludge will have to be introduced.

Aqua Critox supercritical oxidation (SCWO) is a new technology which treats sludge. SCWO can destroy 99.99% of all organic material leaving only clean water and an inert residue. The residue can be used as a construction material or landfilled meaning no land spreading of sludge is needed. The process also allows for the recovery of phosphorus which can be used as an ingredient for fertiliser. This would provide an option should land spreading of sludge become unavailable and would provide an opportunity to offset the peak of phosphorus. The SCWO process will be applied to the dairy industry in the next stage of the thesis to as a possible alternative to land spreading of sludge.

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