A report commissioned by Anglian Water says sludge from Southend is acidic and a health and safety risk to workers once it reaches the treatment works in Stambridge.
Fears have also been raised about its health effects on residents living near the site.
The report, which was written by infrastructure specialists Halcrow in December last year, only became public last week.
It said the time taken for the sludge to travel the 6km from Southend to Stambridge caused a build-up of hydrogen sulphide and has corroded the equipment in the dewatering plant, as well as presented a health risk to workers there.
Anglian Water wants to expand its operation at Stock Road, Southend, to handle Southend waste which is currently sent to Stambridge. Medical equipment company KeyMed, based in Stock Road, opposes the plan.
An Anglian Water spokesman said: "The state of the sewage arriving at Stambridge is a contributory factor, along with cost, in our plans for Southend.
"The footprint of the plant in Stock Road will not change as the new plant will be built on unused land and it is much better at dealing with odour than the older plant is."
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