Impact of kitchen waste grinder application with water conservation strategies on urban water sewerage systems

Abstract

Wastewater discharged from the domestic household has proven to be a potential source for resource recovery. Apart from wastewater, kitchen organic waste contains an enormous amount of energy in terms of organic content and nutrients such as TKN and TPH. Now that the disposal of waste into landfills is prohibited in most parts of Europe, one of the options is to divert the organic waste to wastewater treatment plants using kitchen waste grinders (KWG). Since KWG is a water-consuming appliance, the installation of KWG with water conservation technologies may be a viable option in the future. This can aid efficient recovery of resources and reduce the drinking water demand. The evolution towards sustainable urban sanitation will lead to several positive and negative effects. Some of the positive effects, apart from resource recovery, are the possibility to postpone the enlargement of existing sewer systems, to construct new sewers of smaller pipe diameter and to lower energy consumption for sewage pumping. While the negative effects are blockage of sewer due to reduced flow rates, increased sedimentation and release of malodour in the contemporary sewerage system and increased treatment costs due to increase in COD oxidation and nitrogen or phosphorus removal. Eventually the low flow-high load wastewater needs to be transported through sewers and treated locally (decentralised treatment). In this study, an attempt was made to investigate the effect of coupling the effluent from KWG at various penetration rates such as 100%, 75%, 50% and 25% with water conservation scenarios on hydraulic and quality parameters in the sewerage system. Water conservation scenarios were onsite greywater and rainwater reuse and application of ultra-low water demand appliances. SIMDEUM was upgraded with the addition of an extra appliance in the form of KWG to generate stochastic discharge patterns with appliance specific wastewater flows and quality. The data generated was incorporated into the sewer network model, InfoWorks ICM to analyze the impact of the addition of KWG with water conservation strategies on the contemporary sewer network. The results obtained showed that there is an increase in the mass load of COD, TKN and TPH of 118%, 84% and 90% respectively and reduction in flow, velocity and shear stress of up to 54%, 49% and 74% respectively for the application of water conservation strategies with KWG. This substantiated the fact that the contemporary sewer system is not efficient in transporting the wastewater generated due to the addition of KWG along with water conservation strategies at the household level. A new sewer design with smaller diameters and steep slopes was used to study if the low flow-high load wastewater can be safely transported, in terms of adequate self-cleansing velocity and shear stress. The results from the investigation of new sewer design showed that 100% and 75% implementation of KWG with water conservation technologies will achieve self-cleansing capacity which will help prevent clogging and sedimentation for a piping network of 110mm diameter at a slope of 1:160. KWG market penetration rates of lower than 50% along with water conservation strategies might face issues of sedimentation and clogging. In conclusion, the up-gradation of SIMDUEM with KWG was successful to generate stochastic discharge patterns. This research has demonstrated that the application of KWG along with water conservation strategies may aid in resource recovery and reducing freshwater demand in the urban water cycle. The accomplishment of implementation of KWG with water conservation strategies is not realistic in the contemporary sanitary system however, this transition can be adopted in new urban developments with efficient design of wastewater transport and decentralized treatment system. Further research is required into the feasibility of a decentralised resource recovery system for the implementation of KWG with water conservation strategies.