# Water Consumption, Quantity and Quality of Wastewater and Sewage Sludge from Polish Dairies
## 3 Results and Discussion
In three of the four dairies where specific water consumption exceeded 3.0 L/L, the production profile was very diverse and included the production of ripening cheese, cottage cheese, butter, fermented beverages, yoghurt, buttermilk, kefir, cream, milk powder, whey powder, etc. Such fragmentation of production is due to the use of many smaller specialized production lines and consequently to higher specific water consumption. Particularly high water consumption (up to 17.2 L per L) is typical of fermented milk production [10]. Whey processing also has an impact on the amount of water consumed [37; 38]. Thus, it may be expected that the specific water consumption per litre of processed milk will be lower in dairies processing whey. The analysis of data showed no explicit influence of whey processing on the increase in the volume of water per unit compared to the plants not processing whey. In the plant where the unit volume of water was the lowest (1.5 L/L), 0.8 million litres of whey were processed in addition to 1.1 million litres of milk (80% of the processed milk volume). At the same time, in the plant with the highest specific water consumption (3.71 L/L), the proportion of whey processed was less than 50% (Figure 1). This confirms that, in addition to independent factors such as the amount of milk and whey processed, water consumption is also determined by the individual solutions implemented in a particular plant [39]. Based on the data obtained, the volume of water consumed depending on the production type could only be determined for the blue cheese and ripening cheese production plants. For the maturing cheeses, the volume of water used per litre of processed milk was between 1.75 and 2.37 L/L, whereas for blue cheese it was 1.7 L/L (Table 1, Figure 1; processed milk of 256,200). For both types of cheese, the water consumption values were below the upper limit specified in the literature (0.24-4.90 L/L) [5]. As no information was available on the production profile of the other diaries or the product range was very broad, it was not possible to determine the specific water consumption in the production of other dairy products. Most dairies surveyed had their own WWTPs (13). Those that did not have such a plant (5) discharged their wastewater into the municipal sewage system. Three plants had no facilities for pre-treatment of wastewater prior to discharge into the sewage system, while the remaining two plants had facilities for mechanical and chemical pre-treatment of wastewater or for biological aerobic pre-treatment of wastewater in activated sludge chambers (Tables 1 and 2). The diaries differ in the amount of wastewater produced during the production processes associated with the processing of 1 L of milk. The lowest value of this indicator is 1.18 L per L of milk, while the highest value is 5.78 L per L (Figure 2). Nine diaries produced more than 3 L of wastewater when processing 1 L of milk. These are high values compared to the data from the 1970s (2.43-3.14 L/L; [40]) and 1990s (1.26 L/L; [6]). At the same time, they are also higher than the values calculated by [PERSON] et al. [2] for drinking milk, ripening cheese and milk powder, namely 0.3-3 L/L milk, 0.75-2.5 L/L and 1.2-2.7 L/L, respectively. Figure 1: Unit water consumption in diaries depending on the quantity of milk processed. “*” the volume of processed whey. In some plants, especially those that process whey, a higher volume of generated wastewater may result from the discharge of permeate from membrane whey thickening into the sewerage system as well as from milk and whey losses during production processes and their discharge into the wastewater system. The parameter that shows that such processes can take place is the unit quantity of wastewater per 1 L of water consumption (L/L). Only in two plants was the volume of wastewater discharged lower than the amount of water consumed (50 and 82%), which could be due to very economical water management (Figure 3).