Combined activated sludge and sand filtration for purification of UASB effluent with high suspended solids from water hyacinth juice

Rapid biogasification of water hyacinth, a globally overgrown species, using compression and up-flow anaerobic sludge blanket (UASB) treatment of the juice has recently been proposed. This study aimed to establish a post-purification system for UASB-treated juice to effectively utilize the remaining nutrients for valuable products, such as vegetables and microalgae. To easily remove hard-to-degrade suspended solids (SS) and organic matter from UASB-treated juice, the activated sludge (AS) and sand filtration processes—conventional biological treatments—were introduced, with a focus on their physical treatment properties. Over 76 % of SS and 43 % of the organic carbon were removed from UASB-treated juice during the process. The hydraulic retention time of the AS was reduced to one day, indicating the potential of minimizing the facility size. The light transmittance, which is crucial for microalgae mediums, improved sevenfold. Even after most of the SS was removed by the AS, the sand filtration further increased the light transmittance by 1.4 times. The AS effectively removed most of the SS and organic matter, whereas sand filtration enhanced the treatment stability and further improved the light transmittance. NH₄⁺ was mostly oxidized to NO₃^-, which is less toxic to plants. The total inorganic nitrogen content remained high after treatment, suggesting that UASB effluents from water hyacinth can be used as a nutrient source for hydroponics and microalgae cultivation in developing countries.