This document contains information about effluent washwater discharges associated with the operation of exhaust gas cleaning systems (EGCSs), also known as sulfur oxide (SOx) scrubbers, onboard vessels. The use of scrubbers to clean the exhaust from marine engines using high sulfur residual oil and diesel fuels is an option for reducing SOx air emissions required by Annex VI of the MARPOL International Convention for the Prevention of Pollution from Ships. Washwater generated by an EGCS can contain contaminants from three sources: 1. Pollutants scavenged from the exhaust gas exiting the engine (combustion products, fuel and lubricants); 2. The source of washwater used to clean the exhaust (seawater or freshwater); and 3. The scrubber itself (dissolution of materials, possible reaction products and/or chemical additives). The deployment of SOx scrubbers to treat emissions from diesel engines on large ocean going vessels has been so far limited to a handful of ships. However, the use of SOx scrubbers in combination with high sulfur residual oil fuels may be an economically attractive option in Sulfur Oxide Emission Control Areas (SECAs) and worldwide. As the deadlines for SOx reduction1 draw nearer, some vessels may begin using scrubbers to reduce SOx emissions rather than burning low sulfur fuel oil. This document describes the basic technology of EGCS, treatment processes used to remove pollutants from washwater prior to discharge, guidelines established by the International Maritime Organization (IMO) for washwater monitoring and discharge criteria, available onboard sampling data characterizing the constituent concentrations in washwater discharge, and impacts of scrubber washwater discharge on water quality and aquatic life and protectiveness of IMO guidelines.