**4. Discussion**

Assuming that the waterworks sludge is virtually inert and does not degrade in the digestion chamber, the entire dry content of the added waterworks sludge will pass through the digestion chamber and be present in the residual biosolids. Based on the information collected from the examined biogas plants included in the study, the waterworks sludge contributes to an increased amount of digestate; that is, about 1–3% based on dry matter, i.e., about 1–3% of TS. The digestate in most plants has a TS content of around 5%, while the waterworks sludge has a TS content of around 15%. The volume increase in the digestate to be handled due to the addition of waterworks sludge is 1% at the most; the TS content in the digestion increases by about 0.1–0.3%, while the total amount of metals in the digestate increases by approximately 1–3% (see calculation in Table 5). Table 5 also refers to the quality requirements according to the biofertilizer certification system, SPCR 120 [12]. This standard states that the proportion of each of the metals (lead, cadmium, copper, chromium, mercury and zinc) may not exceed 15% of the total amount of the metal in the certified biofertilizer. In addition, the nickel content in the biofertilizer from the waterworks sludge must not exceed 6 mg/kg biofertilizer (wet weight). In 2019, around 2.8 million tons of digestate (wet weight) was produced in Sweden, of which 87% was used as fertilizer in agriculture [19]. From farm plants and co-digestion plants, 2.13 million tons, virtually all digestate (biofertilizer), was used as fertilizer. From the wastewater treatment plants, 0.25 million tons (41% of all the sludge) was used as fertilizer certified in the Revaq system. The remaining amount was used mainly as construction material or for the final coverage of landfills [19]. No biosolids are generated at landfill gas plants.

The biogas production from the other plants that also generate biosolids corresponds to a total of 1969 GWh [17]. These plants simultaneously produced 2.8 million tons of biosolids, or around 700 kWh/ton of biofertilizer. Assuming that 1 Nm<sup>3</sup> CH4 corresponds to 9.81 kWh [21] and that the biogas contains 65% methane [22], this corresponds to a production of 110 Nm<sup>3</sup> of biogas per ton of biofertilizer. According to the values for the 13 plants using waterworks sludge from Ringsjöverket surveyed above, around 1 ton of waterworks sludge is added per 8750 Nm<sup>3</sup> of biogas that has been produced. This corresponds to adding 1 ton of waterworks sludge per 108 tons of biofertilizer produced in the plant, excluding the added waterworks sludge, or 9.2 kg of waterworks sludge per ton of biofertilizer produced. Table 5 shows how adding waterworks sludge affects the concentration of metals in the biofertilizer. The starting point for the calculation is the average value of the metal contents in the biofertilizer for the 18 plants that were certified within SPCR 120 in 2014 [23]. Nickel is not included as there is no risk that the content of nickel can reach up to 6 mg/kg wet weight given the concentration of nickel present in the waterworks sludge. Among other heavy metals, lead and chromium make up the largest parts of the total metal concentration, at 7 and 6%, respectively. However, this is below the limit of 15% specified according to SPCR 120. Therefore, this is also not considered to be a problem for the use of waterworks sludge. With the addition of the waterworks sludge, which is assumed to be inert, the S content of the biofertilizer increases. Assuming the same waterworks sludge dose as above, the TS in the biofertilizer increases from 3.9 to 4.0%. An effect of this is that the concentration of metals, stated as mg/kg TS, decreases for, e.g., copper and zinc, while the concentration increases slightly for lead and chromium (see Table 5).

**Table 5.** Average concentration of heavy metals in biofertilizer for the facilities that were certified in 2014 [23] before and after an estimated addition of waterworks sludge. The TS in the biofertilizer is 3.9% before the addition of waterworks sludge. The concentration in the sludge is the average of analyzed data according to Table 3.


Since about half of the waterworks sludge consists of various organic compounds from lake-source water, the waterworks sludge contributes to the increased organic content in soil where the digestate is spread as fertilizer. In many Swedish soils, the organic content is low, which is why this is a welcome contribution to improving the soil's properties. The organic content in soils improves the physical, chemical and biological properties of the soil, such as its water holding capacity, nutrient content, buffer capacity, and the activity of soil organisms.

To reuse material is beneficial for climate and society, and reduces the carbon footprint. According to the evaluation in Sydvatten's Climate Account Report 2020, the production of virgin iron chloride generates about 0.395 kg CO2 emission per kg FeCl3. Since Sydvatten used 3132 tons of FeCl3 in 2020 for drinking water treatment, the reuse of 56% sludge by replacing virgin ferric chloride with waterworks sludge would eliminate 740 tons of carbon dioxide, which is about 17% of all the carbon dioxide that was emitted by the company in 2020 [9], and is well in accordance with EU's Circular Economy Plan [24].
