Dirty Water Coolers and Mock Sewage Pipes

Dirty Water Cooler
Look for a second hand water dispenser on eBay or Gumtree. Fill it with a murky non-toxic liquid and add a very visible company logo. Then offer employees and onlookers a taste of the finest sewage-laced beverage, courtesy of the target water company.
Mock Sewage Pipe
This may require a trip to your local arts & crafts store (or re-cycle one of those hand-chain-tubes). Create some paper & fabric effluent, add a company logo and a shut-off valve for some more effect. Ideally a 'company executive' joins the installation for regular spewing of creative sewage.