All waste of that type collected commercially must be taken to a designated site, the Water Co. will tell you where will accept septic waste imports. You will probably be issued with a swipe card to allow access to the site, which will then be required to log the load in, and open the valve on the discharge point to permit you to tip. The system records the volume of waste tipped and the Water Co. will bill you accordingly.
Tipping anything down a manhole will likely result in prosecution if/when you are found out. Unscrupulous operators use the drainage system as a cheap method to illegally dump hazardous wastes, which then have a knock on effect at the treatment works. Most incidents of this type can and are traced back to the point of origin, and if you are found responsible, expect to find yourself in court.
Sewage Treatment relies heavily on bacterial processes, if these bacteria are killed off then the whole process stops, untreated effluent can be discharged to the environment and there is one hell of a bill to sort it out.
