Decision making tool for industrial dairy sludge management : a real case study

Abstract

Uruguay, a small country with a population of 3 million people, is the seventh largest exporter of dairy products in the world. Also, seventy percent of the milk produced in Uruguay is exported. Uruguay's commitment to sustainable practices can be reflected in the country brand “Uruguay Natural", which emphasize a genuine connection with nature. In this context, the dairy sector faces the challenge of properly managing its waste, from primary production to consumption, including industrial processes. An important type of high-organic-content waste generated is the sludge from the wastewater treatment systems of dairy processing plants, which produce liquid milk, powdered milk, and a significant range of mass- consumption products. Currently, the most common method of managing these sludges in Uruguay is their disposal in a landfill. However, this option is neither the most suitable for minimizing environmental impact nor the most cost-effective one. There are different alternatives for the disposal of these sludges, considering the sustainability of the entire value chain. Some of them include the distribution of digested sludges on dairy farms as soil amendment, composting, use as an energy source, or as a partial substitute for nutrients after drying and pelletization, among others. However, the environmentally proper management of these sludges often entails significant costs for companies, whose finances are strained by intense international competition. How to select the most suitable path to minimize economic impact? This question resumes the aim of this work. It is addressed within the context of a case study involving an industrial plant located in the central region of the country. Limited attention has been devoted to the application of mathematical optimization tools specifically for the management of sludge within the dairy industry (Perimal et al. (2017), Yapiciočlu and Yeşilnacar (2021), Olajire and Shah (2009)). This research aims to develop a decision-making framework tailored for the board within the dairy industry, facilitating comparative analyses across diverse scenarios, that is, to solve a process synthesis problem (PSP) of sludge management within the framework of Process Systems Engineering (PSE). Different management alternatives were considered, with the selection criterion being the possibility of obtaining experimental data on efficiencies and costs at a real scale: combinations of chemical conditioning and mechanical dewatering, drying, on-field distribution as a soil amendment in dairy farms, use as fuel and disposal in sanitary landfills as a base case for comparison. The problem is implemented for a specific case, but by adjusting parameters related to geographic location, site-specific costs, and characteristics of the sludge to be disposed of, the tool is applicable to other cases. Predicting the mechanical dewatering efficiency of a candidate technology is challenging, as this parameter depends on the type of sludge under consideration. This aspect is significant because the residual moisture content in sludge cakes impacts subsequent treatment costs due to increased volume. Thus, it is important to identify the primary variables within the dewatering process (Novak (2006), Richard I. Dick and Novak (1980), WAK (2006)). A second objective is to assess dewatering efficiencies for sludges of different origins, providing valuable information to extrapolate the results obtained in this case to other dairy plants.