After the completion of the first phase of works for sewerage and the installation of the first stage treatment of sewage in 1992, no more than 25% of city sewage were channeled for treatment, while up to the year of 2000 the rest 75% of urban and industrial sewage of Thessaloniki were channeled into the sea, without any treatment.
But only after additional interventions in the sewage network, the operation of the installation for treatment of sewage of Tourist regions in Michaniona and mostly after the completion and the complete operation of the installation for second phase treatment of sewage of Thessaloniki, essential conditions were introduced in favor of the quality of the coastal seawaters, confronting the major source of organic pollution.
Up to the year of 2008, the periodical monitoring and follow-up of water quality of Thermaikos was assigned to the Institute of Oceanography of Greek Centre of Marine Researches. In that way, the degree and the rates of measurable changes in water quality were recorded and watched by means of explicit indicators. Moreover, the water clarity in the gulf significantly depends on additional anthropogenic activities such as the pollution from passing boats etc., and over and above surges of either anthropogenic pollutants or drifted earth matter, flowing into the gulf via several local streams and rivers.