# Water Consumption, Quantity and Quality of Wastewater and Sewage Sludge from Polish Dairies
## 1 Introduction
The benefits of this solution have been outlined in numerous publications [15; 16; 17]. Sludge application promotes plant growth due to the high concentrations of phosphorus, nitrogen and carbon and micronutrients (K, Cu, Fe, Mg, Mo and Zn), as well the low content of toxic metals. The application of dairy sewage sludge in agriculture can be legal, as the accumulation of heavy metals in this type of sewage sludge poses no problem [15; 18]. The data on the quality of sewage sludge obtained in the studies carried out in the early 2000s do not reflect the characteristics of the sludge currently being disposed of from dairy WWTPs. This is due to the fact that in the last 15 years, DAF devices have been installed in many WWTPs to reduce the pollutant load of organic compounds, fats, oils and greases (FOGs) that flow into the biological section of the treatment plant. This change in treatment technology had an impact on the quantity and quality of the sludge removed from the biological part of the treatment plant. At the same time, sludge began to accumulate in the mechanical part of the treatment plant due to the flotation process, which contained significant amounts of poorly degradable FOG. From then on, a mixture of mainly lime-treated DAF sludge and biochemically treated excess activated sludge was discharged from WWTPs [13]. The continuous modernization of Polish dairies affects water and wastewater management in the dairies and consequently modifies the quantity and quality of wastewater discharged into the dairies' wastewater networks and sewage sludge from WWTPs. According to data from the Ministry of Climate and Environment (letter dated 16 February 2024) [19], based on the BDO (Database of Products and Packaging and Waste Management), the annual amount of sewage sludge (waste code 02 05 02--s sludge from on-site WWTPs in the dairy industry) in Polish dairies in 2020, 2021 and 2022 amounted to 95,352, 90,642 and 81,626 Mg dry matter, respectively. The observed downward trend is due, among other things, to the increasingly widespread replacement of aerobic by anaerobic technologies, both in terms of wastewater pipes and sewage sludge treatment plants in dairies. The second factor affecting the mass of sludge produced is the modernization of some biological WWTPs, which leads to a reduction in the pollutant load of the activated sludge, resulting in lower sludge growth and significant stabilization of the sludge [13]. There is also little data on the quality of dairy wastewater from Polish dairies. The last comprehensive wastewater tests were carried out in the early 2000s. [10]. It should be noted that, according to the I Statistic Poland, milk purchases in Poland increased by over 3 billion litres or 35% between 2005 and 2023 [20]. It is therefore crucial to keep up-to-date data on water consumption and the quantity and concentration of dairy wastewater discharged into the sewage system. There is also a gap in the literature regarding the specific amount of wastewater per litre of water used to manufacture dairy products. A significant change in the production range, modernization of facilities, and a significant share of cottage cheese in cheese production all together significant impact on the quality of dairy wastewater and its quantity. Changes in sludge management in dairy WWTPs also require determining the ratio of sludge to raw milk processing and the amount of sludge produced per pollutant load removed (true growth yield, Y). The values of these parameters will be useful for both planners and operators of WWTPs. The aim of this study was to determine the volume of water consumed and the volume and quality of wastewater and sludges discharged from dairies in Poland in 2023 based on the size and production profile of the plant.