In 1846, French public hygienist Alphone Guérard estimated that 100 cesspits were cleaned in Paris every night, by 200–250 total cesspit cleaners in the city, and out of a total of 30,000 cesspits. The replacement of Paris' cesspit system was challenged for decades by officials not on public hygiene grounds, but on economic ones, based on the desire to conserve human waste as fertilizer rather than disposing of it in a modern sewer system. Paris' sewer system began modernizing in the 1880s, with the conversion of storm sewers for public sewage. Some cesspits were still in use in Paris into the 20th century.