Aiming to promote low-cost production of protein-rich forage rice and resource recycling from wastewater treatment plants, a pot experiment was conducted to assess the possibility to substitute mineral fertilizers with composted sewage sludge (CSS) with/without top-dressing with treated municipal wastewater (TWW). Results indicated that a basal application of CSS at 2.6 g N pot-1 replaced conventional mineral fertilization of 1.3 g N pot-1 to produce comparable yields with the same rice protein content, although there might be a risk of increased As concentration in rice grains. Interestingly, CSS application at a reasonable dose of 1.3 g N pot-1, followed by a topdressing with TWW resulted in 27% higher yield and 25% superior rice protein content relative to the mineral fertilization, with no risk of heavy metal(loid) accumulation in grains and in paddy soils. Here we demonstrated an appealing fertilization practice with zero use of mineral fertilizers in paddy rice cultivation, expectedly contributing towards sustainable rice farming and animal husbandry in Japan.