Shortly after his removal to London he became interested in the sewage problem of the Metropolis, and was called upon to advise the Metropolitan Board of Works with respect to methods for improving the condition of the river Thames. In connection with this work he established a laboratory, specially equipped for studying its problems, in the Earl’s Court Road, not far from his London residence. During the year 1887 he was engaged, with the assistance of his former pupil Mr. Harry Baker, in reporting to Lord Magheramorne, the Chairman of the Metropolitan Board of Works, on the chemical methods employed for the deodorization of sewage (_a_) in the metropolitan sewers, and (_b_) at the outfalls. Early in 1888 much larger problems were submitted to him: viz. the purification of the sewage, the disposal of the sludge, and the effect of the discharge of sewage sludge at sea on the foreshores of the estuary of the Thames. In connection with these subjects he became impressed with the importance of obtaining accurate scientific methods for determining the changes which polluted water experiences during its natural purification. Some of the results of his inquiries he published in conjunction with his pupil Mr. Joseph Lunt in two memoirs, one “On Schützenberger’s Process for the Estimation of Dissolved Oxygen in Water,” communicated to the Chemical Society in 1889, and published in the _Transactions_,[28] and the other entitled “Contributions to the Chemical Bacteriology of Sewage,” which appeared in the _Philosophical Transactions of the Royal Society_.[29] The former paper contained the results of a careful investigation of the conditions under which this method is alone trustworthy, and served to explain the causes of the discrepancy between the statements of previous observers who had critically examined it. The latter paper gave the results of a protracted examination of the chemical and bacteriological phenomena of crude sewage with the object of ascertaining the species of organisms present, both pathogenic and saprophytic, and of determining their chemical characteristics. These investigations were carried on for more than two years, concurrently with the technical and outside work required. During this time purification works had been established at Crossness and Barking outfalls, a sludge ship had been provided for the disposal of the sewage sludge at sea, and the effect of the discharge had been studied in the lower reaches and estuary of the Thames, and a chemical survey of the condition of the foreshores had been completed. But the formation of the London County Council, with Lord Rosebery as the first Chairman, involved new arrangements. This circumstance, combined with the death of Sir Joseph Bazalgette, the Chief Engineer, and the opposition of the Labour Party, resulted in Roscoe resigning his post as Scientific Adviser.