Byproducts of thermal hydrolysis include soluble coloured refractory compounds (melanoidins). Their presence in dewatering liquors returned to the WwTW can result in an increase in effluent COD concentrations. At THP sites where much of the sludge is imported, the increase in final effluent COD can be significant, raising concerns about compliance with the UWWTD limit (125 mgCOD/l). Historical data from multiple THP sites in the UK and Europe was used to quantify the impact of THP on the effluent COD concentration and to establish the relationship between the effluent COD load increase and THP throughput where possible. The data indicates that on sites treating indigenous sludge only, the impact on effluent COD concentration is small or non-detectable. Effluent COD is impacted when substantial amounts of external sludge are treated in a centralised treatment facility. The conclusions of this data analysis were then applied to Scottish Water’s planned Daldowie project, which will treat sludge from Greater Glasgow and Central Scotland, allowing an assessment of the risk to continued compliance with effluent quality and to evaluate the need for mitigating actions.