# Water Consumption, Quantity and Quality of Wastewater and Sewage Sludge from Polish Dairies
## 1 Introduction  
In 2023, the EU Member States produced around 160.8 million tonnes of milk [1], with Poland accounting for 15.2 million tonnes in 2024. In 2019, with an annual EU production of 167.4 million tonnes, dairies produced 192.5 million m\({}^{3}\) of dairy wastewater, which corresponds to a specific wastewater volume of 1.15 L per L of processed milk. Poland is one of the six largest milk producers in Europe and is responsible for discharging 73% of the milk wastewater produced in the EU into the sewerage system. Based on calculations made by [PERSON] et al. [2], it was estimated that Polish dairies would produce 16.95 million m\({}^{3}\) of wastewater in 2024. To estimate the amount of dairy wastewater generated in the EU-27 countries, a Greek research team analysed the annual production volumes of milk, cheese, butter and fermented milk in the individual countries [3]. Using data from the European DairyAssociation (EDA) [4] on the specific volume of wastewater per kilogramme of dairy products, they calculated the quantities of wastewater produced by the European dairy industry [2]. Three types of wastewater are produced in dairies: process wastewater, domestic wastewater and cooling water and condensates [5]. The quantity and quality of process wastewater depends primarily on the plant size, with variable concentrations and irregular flow rates influenced by the number of production shifts and production type. Cheese and butter production plants generate most of the wastewater [6; 7; 8]. In the EU, 49% of wastewater comes from cheese production, and the same is true for Poland [2]. The structure of production in Poland is dominated by cottage cheese (52%), followed by cheese matured with rennet (39%) and processed cheese (8%) [9]. The special nature of the wastewater produced in Polish dairies is due to the frequency of cottage cheese (tvarg) production. Polish cottage cheese is unknown in other countries. The quality of dairy wastewater is influenced also by the amount of raw materials lost during production, the management of whey, the use of permeate, the purification methods deployed and the type of detergents and disinfectants used [10]. Milk and fat losses in cheese production amount to 0.2% and 0.1%, respectively, while these losses in the production of drinking milk reach 1.9% and 0.7% [5]. A specific by-product of cottage cheese production is acid whey. It usually makes up 75-80% of the processed milk mass. A major problem is the management of acid whey: when discharged into a WWTP, it causes a significant reduction in the pH of the wastewater [11]. Sewage sludge is produced during the treatment of dairy wastewater (dairy processing sludge). Some of it is produced in the mechanical part of the treatment plant (mainly dissolved air flotation processing sludge (DAF)), but most of it is produced in the biological treatment plants, operating with both oxygen and anaerobic technology [12]. The estimated annual wastewater volume of 16.95 million m\({}^{3}\) for Polish dairies, which is based on the methodology of [PERSON] et al. [2], is an approximate figure that does not come from direct measurements in the plants. There is a lack of data in the literature on the actual volume of wastewater produced by Polish dairies, taking into account the size and range of production. Similarly, there are no specific data on the water consumption per litre of milk processed in the production processes or the volume of wastewater discharged into the sewage system per litre of milk processed [13]. According to [PERSON] et al. [14], about 2 kg of sludge is produced per 100 L of milk processed. Thus, considerable quantities of sewage sludge from the dairy industry have to be dealt with at a national level. The most common solution is its agricultural and natural use after prior stabilization in aerobic and anaerobic sludge neutralization plants. Spreading on agricultural land is a common practice not only in Poland but also throughout Europe [12].