mercaptan, dimethyl sulfide, dimethyl disulfide, and other volatile sulfur compounds are the cause of the malodorous air emissions characteristic for pulp mills utilizing the kraft process. The sulfur dioxide emissions of kraft-pulp mills are much lower than those from sulfite mills. In the ambient air outside a typical modern kraft-pulp mill, the sulfur-dioxide odour is perceivable only during disturbance situations, for example when the mill is shut down for a maintenance break, or when an extended power outage occurs. Control of odours is achieved through the collection and burning of these odorous gases in the recovery boiler alongside the black liquor. In modern mills, where well-dried solids are burned in the recovery boiler, hardly any sulfur dioxide leaves the boiler. At high boiler temperatures, the sodium released from the black liquor droplets reacts with sulfur dioxide, thereby effectively scavenging it by forming odourless sodium sulfate crystals. Pulp mills are almost always located near large bodies of water due to their substantial demand for water. Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes. The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. The process effluents can be treated in a biological effluent treatment plant, which can substantially reduce their toxicity. See also H-factor Johan Richter – Inventor of the continuous process for digesting wood pulp Kappa number Kraft paper Organosolv Paper Paper chemicals Pulp mill Soda pulping Wood pulp References Further reading External links Pulp and Paper Circle provides industry-related information (especially Kraft Mills) to pulp and paper engineers and technologists. US EPA article on Kraft pulping Reference Document on Best Available Techniques in Pulp and Paper Industry by European Commission, 2001 Papermaking Chemical processes Pulp and paper industry 1879 in science 1879 in Germany