The largest of the contracts, worth £31m, will ensure that Blackburn Meadows meets the requirements of the EU’s Freshwater Fish Directive and will take 18 months to complete.
The plant overhaul will ensure that water pumped back into the Don can meet new, even tighter European standards.
The refurbishment is designed to reduce levels of ammonia in the river for the benefit of fish and other wildlife. Ammonia is found in a range of cleaning products and detergents, as well as in human urine.
The second scheme is worth £18m and will result in a new mesophilic anaerobic digestion plant.
Greg Fitzgerald, Chief Executive of Galliford Try, 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.”