Proof of concept design of a black water disinfection system for on board use on recreational vessels and yachts

Abstract

There are more than 160,000 recreational crafts in the Netherlands with a toilet on board. It is forbidden to discharge blackwater into the environment from these crafts. Stricter enforcement of the law by the sealing of the valves of the external drain, leaves the owners of these crafts with a major problem. It is allowed, however, to treat the blackwater on board and then discharge it if the following two boundary conditions for fecal indicators are met: a maximum concentration of 330 cfu per100 ml of Intestinal enterococci and a maximum concentration of 900 cfu per 100 ml of E. coli. The overall objective of this research was to find and test a proof-of-concept (PoC) method to reach these boundary conditions. The method consisted of a series of conventional wastewater techniques: first a coagulation and flocculation step, then the filtration and subsequent dewatering of the TSS and lastly the filtrate was disinfected using UV-C irradiation. Bench scale experimental setups were used to get the data for this research. Results demonstrated that TSS was most efficiently removed by a combination of polyaluminnium chloride and a cationic polyacrylamide copolymer with a low charge density. Furthermore, polyaluminium chloride demonstrated to be highly effective at removing the fecal indicators. Dewatering experiments proved that increasing the polymer dosage did not affect the total dry solid content of the final sludge cake. Also, under the most ideal circumstances a dry solid content of the filter cake of 30% was achieved. Lastly, UV-C experiments were conducted on the filtrates of two different samples, with values for UV254 absorbance of 1.34 cm−1 and 11.85 cm−1 respectively. The first of which was sufficiently disinfected after a total irradiated UV-C dose of 180 mJ/cm2, of which the effective dose was 54.9 mJ/cm2. The fecal indicators in the second sample were not sufficiently removed after a total UV-C dose of 1100 mJ/cm2. The absorbance of UV254 of the filtrate was so large that the effective dose was only 40.0 mJ/cm2. The PoC method showed to have potential for implementation, but adjustments need to be made to make it more sufficient and viable for use of on board disinfection of black-water. Preliminary screening can extract most of the fecal material, which is already high in dry solid content, before homogenising with the liquid fraction. The quantity of conditioning agents need to be adapted to the specific blackwater, and if necessary the pH will need to be adjusted to ensure the right coagulation mechanism for efficient TSS removal. Finally, humic matter in the filtrate will have to be reduced for UV-disinfection to be an effective treatment method. This could possibly be achieved by mechanisms such as adsorptive coagulation or by implementing an intermediate ozonation step after filtration of the TSS, which would also help in the removal of fecal indicators.