Work is divided into two contracts. The larger is worth £31m and, over the course of 18 months, will bring Blackburn Meadows up to the standard required by the EU's Freshwater Fish Directive.
The second scheme, worth £18m, is a new mesophilic anaerobic digestion plant.
Chief executive Greg Fitzgerald said: "We are delighted to secure such substantial and significant schemes from one of our longest standing clients. We have considerable expertise and experience of anaerobic digestion from other water clients and we're glad to have the opportunity to share this with Yorkshire Water."
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