The focus of valorization in this study was to rapidly degrade sludge wastes from an RAS into soluble materials, remove nitrogen and obtain more organic acids to be used for denitrification later on. The ammonium nitrogen produced during valorization is not desirable in the operation of an RAS due to its toxicity to fish, and nitrate is also undesirable since it can consume electron donors (e.g., organic acids, etc.) that should be reused for denitrification in an RAS. A higher removal efficiency of ammonium-N (54%, 3.7 mg N L−1 d−1) was shown 12 days after the KBM-1 treatment compared with that of the control (7%, 0.6 mg N L-1 d−1) eight days after valorization. The ammonia-N might have been removed through nitrification. However, higher removal efficiencies of nitrate (NO3- -N) (27–41%; 1.5–4.5 mg N L−1 d−1) were observed two to four days after treatment with KBM-1, while lower removal efficiencies (13–16%, 0.5–1.3 mg N L-1 d−1) were shown in the control after four to eight days, indicating the effectiveness of KBM-1 treatment for nitrate removal. In this case, the organic acid production and denitrification rates were the highest after two days. Therefore, the nitrate seemed to be removed through denitrification utilizing the organic acids as electron donors. Considerable waste reduction can occur through the production of mainly gaseous carbon and nitrogen by biodegradation, in which heterotrophic microorganisms and denitrifiers play an important role. For example, the loss of nitrogen is mainly a result of denitrification in the oxygen-depleted zones in the system and may account for as much as 21% of the gaseous nitrogen loss in some RASs (van Rijn et al., 2006). Some indoor RASs, where ammonia is nitrified to nitrate, employ special reactors to induce the bacterial reduction of nitrate to nitrogen gas under anoxic conditions where nitrate is used as an electron acceptor instead of oxygen (van Rijn, 2013). Sludge degradation reaching 30–40% was reported for denitrifying reactors fed with marine RAS effluents and operated at short retention times of up to 11 days (Klas et al., 2006). Moreover, it was shown that a hydraulic retention time of eight days can achieve a 74% reduction in organic matter and a total reduction of nitrogen with this kind a treatment scheme. RASs incorporating sludge digestion and denitrification may be operated with little or no effluent discharge because much of the waste is converted to gases. Fast decay of sludge in the presence of oxygen coincides with fast growth in the heterotrophic biomass of the microorganisms involved in sludge decay. The aerobic degradation constants of “fresh” sludge ranged from 0.07 to 0.40 day−1 (Chen et al., 1997). In reactors operated at longer retention times in which, in addition to oxygen, additional electron acceptors are respired, the decay of sludge proceeds at lower rates than under aerobic conditions and produces less heterotrophic bacterial biomass. Sludge decay constants ranged from 0.006 to 0.024 day−1 in a reactor operated with a high sludge age with nitrate as the main electron acceptor (van Rijn 1995).