**3. Results**

At Sydvatten's Ringsjöverket waterworks in Stehag, approximately 9000 tons of sludge with 15% total solids (TS) are produced annually (see Table 3). The sludge is formed in the chemical precipitate, which is the first part of the waterworks process. The raw water comes from Lake Bolmen in Småland, and the organic content of Bolmen's water is virtually inert, i.e., it will not contribute to the biogas production in the digestion chamber. Today, ferric chloride is used as a coagulation chemical to coagulate organic material in the water. The water pH is corrected with lye before the addition of the coagulant. The sludge settles in lamella sedimentation and is then thickened in a gravity thickener after the addition of iron chloride, lime water and polymer, so as to reach a dry content of about 2%. The sludge is then pumped into a sludge handling plant where it is pressed in sieve belt presses after the addition of additional polymer and iron chloride to a dry content of about 15%. The sludge is finally landfilled in a closed peat extraction area. Data on the overall generation of sludge and the fraction used in biogas plants are presented in Table 3.


**Table 3.** Waterworks sludge generation and fraction used for sulfide removal in biogas plants (Sydvatten, internal statistics).

There are operating situations at the waterworks when no iron-containing sludge is produced. This occurs especially if there is a landslide in the tunnel that runs between Lake Bolmen in Småland and Ringsjöverket. This has historically occurred on three occasions over the past 30 years, and then the tunnel has had to be drained and renovated. On these occasions, water from Ringsjön is used at the waterworks and then an aluminum-based precipitation chemical is used instead of iron chloride. The sludge formed then cannot be used in the biogas industry. These malfunctions usually take between one and two years to rectify. It is therefore important for biogas producers who choose to use the waterworks sludge to be able to switch quickly to an alternative solution, such as ferric chloride, during such a period.

The waterworks sludge from Ringsjöverket has a black-brown appearance (see Figure 1), and if it has been in contact with air for a while, small black iron crystals form on the surface. The sludge is water-soluble and has a slight iron odor. Its density is around 1.1 kg/dm3, and the sludge is slightly acidic, with a pH value around 4.2. The sludge consists mostly of iron compounds (about 30% iron and >40% iron oxides) and various organic compounds (about 25% TOC). In the waterworks, before the sludge is separated, only iron chloride and lye are added. Then, a small amount of the drinking water-grade polymer Magnafloc LT22S-DWI is added together with additional iron chloride in both the thickener and the screen belt press to facilitate dewatering [10].

**Figure 1.** Waterworks sludge (**left**) and manure storage (**right**). Photo: Annika Nyberg.

A detailed metal analysis has been carried out on the dewatered sludge from Ringsjöverket on three different occasions. Results for the most relevant metals are reported in Table 4. Table 4 compares the analysis results with the limit values that exist within Avfall Sverige's certification rules for biofertilizers, SPCR 120 [12] and the Swedish Environmental Protection Agency's general guideline values for contaminated land for sensitive land use, published in 2009 [20]. It is clear that the metal content of the waterworks sludge is below these levels by a very good margin for virtually all metals. Only the limit value for arsenic in the sludge is in the same order of magnitude as the Swedish Environmental Protection Agency's guideline value for contaminated soil in sensitive land use. Revaq is a certification system that has the aim of reducing the flow of hazardous substances to treatment plants and creating a sustainable return of plant nutrients by spreading sludge from wastewater treatment plants on arable land [11]. The heavy metals that hold the greatest interest in Revaq and that require the most frequent analysis intervals within Revaq are lead, cadmium, copper, chromium, nickel, zinc, mercury, silver and tin, and analysis results from all of these are included in Table 4. Even in this case, the concentrations are low compared to the guideline values that exist. Table 4 also shows that the iron content is relatively constant during the year, with only small variations. This is important for biogas producers to be able to use a similar dosage for different sludge deliveries.


**Table 4.** Composition of waterworks sludge from Ringsjöverket compared with guideline values from Swedish EPA [20] and the Swedish Waste Association standard SPCR 120 [12] for permissible metal content for biosolids reuse.

> In 2016, waterworks sludge was accepted as an approved additive to the digestion process within the certification system SPCR 120. Therefore, it is now possible for the biogas plants that have this certification to use the waterworks sludge to reduce the hydrogen

sulfide concentration in the biogas. When it comes to certification according to Revaq, the use of the waterworks sludge should only affect the cadmium (Cd)/phosphorus (P) ratio marginally, because the cadmium content is very low. However, the phosphorus content is also low, which means that even a low cadmium concentration can have a negative effect on the Cd/P ratio. The size of the sludge feed is also affected because this is regulated not only by the metal concentration, but also by the amount of a certain metal that may be laid per unit area. This applies in particular to the metals lead, cadmium, chromium, nickel, zinc and mercury. However, the content of these metals is very low, and should only have a minor impact. The rules for the Revaq certification system state, however, that "The certificate holder shall not receive such material that is deemed to adversely affect the quality of sludge, through low nutrient content or high content of contaminants. The 60 trace elements must always be analyzed before receiving a new type of material". This can be a problem for the use of waterworks sludge in Revaq-certified treatment plants, but it is not really different from using ferric chloride to reduce the hydrogen sulfide concentration, as it also contains some other heavy metals.

During the project, 13 biogas producers that use waterworks sludge at the full scale were contacted. The experiences at these facilities have been collected and compiled below under different categories. Some of the results have already been published in a report in Swedish [10].
