*3.2. Other Costs*

In addition to transport, biogas producers face costs for the storage, dosing and cleaning of the equipment used. For most users of waterworks sludge for hydrogen sulfide control, these costs are considered to be marginal and estimated at somewhere between EUR 2 and 10/ton wet waterworks sludge.

Most of the biogas producers who currently use waterworks sludge have previously used iron chloride that they bought from a chemical supplier. The cost of virgin iron chloride is up tp twice as high as the total handling costs of the waterworks sludge. The location of the plant matters. A large transport cost reduces the net savings of operational costs. According to Broberg [7], the cost of hydrogen sulfide reduction with iron chloride and iron oxide is around EUR 0.01–0.02 per Nm<sup>3</sup> in farm biogas plants. The cost of using waterworks sludge ends up in the same order of magnitude if the cost of handling and transport is estimated at approximately EUR 40 per ton. However, several biogas producers have reported substantially lower transport and handling costs, and in addition, they reduced the hydrogen sulfide concentration to lower levels when using the sludge compared to when they used ferric chloride. This supports the conclusion that cost savings of up to 50% are possible, but that this depends on the transport cost. The reason for the further reduction in the hydrogen sulfide concentration with sludge is that they think they can afford to add a surplus of iron to the digester, since the marginal material cost is lower, and thus they can then control the dosage towards a lower hydrogen sulfide concentration in the generated biogas. A lower residual hydrogen sulfide concentration in the biogas increases the life of the power/heating unit and reduces its maintenance needs and costs. If the substrate used has lower sulfate content, the amount of sludge needed per volume of biogas produced is lower. Other benefits that the operators of the biogas production plants have observed is the easier handling of waterworks sludge when dosing compared to the corrosive ferric chloride solution and the corrosive damage to the equipment that this can

lead to. Some of the farm biogas producers have previously used only aeration or aeration in combination with iron additive. Then, the economic gain achieved with the transition to waterworks sludge is smaller because the addition of air is associated with very marginal costs.

#### *3.3. Use of Waterworks Sludge in the Digester*

The number of plants that utilize waterworks sludge from Ringsjöverket has increased since the test started in 2013. In 2020, a total of 24 different plants collected iron-containing waterworks sludge for hydrogen sulfide removal in the digesters. Based on information from seven biogas plants where only manure is used as the substrate, one ton of wet waterworks sludge (with a TS of 15%) is sufficient to produce an average of 2700 Nm<sup>3</sup> of biogas if the hydrogen sulfide concentration in the biogas is to be reduced below 100 ppm. This corresponds to about 0.2–0.5% of the amount of substrate added, expressed as dry matter. The stoichiometric relation between sulfur and iron could be observed at another biogas plant where the residual hydrogen sulfide content was allowed to be higher. This plant was designed to produce biogas with a residual hydrogen sulfide concentration below 300 ppm. An addition of one ton of waterworks sludge to the digester was enough to produce 8000 Nm<sup>3</sup> of biogas with <300 ppm H2S.

In co-digestion plants where a mixture of manure, starch, food waste and slaughterhouse waste is applied, one ton of wet waterworks sludge was sufficient to reduce the hydrogen sulfide concentration in a significantly larger volume of biogas, since the mix of substrate contains less sulfur than pure manure. Five co-digestion plants surveyed in this study dosed less than half the amount of waterworks sludge into the substrate compared with the manure-based biogas plants, and could still generate biogas with less than 100 ppm hydrogen sulfide. If the proportion of manure dominates in the substrate, the required addition of waterworks sludge remains high, since that kind of substrate is similar to pure manure. The iron in waterworks sludge is less available compared to the addition of pure ferric chloride solutions to control the hydrogen sulfide concentration. When comparing the addition of iron from waterworks sludge with ferric chloride solution, the total amount of iron added to the substrate had to be increased 2.5 to 3 times if added as waterworks sludge in order to achieve a similar effect on hydrogen sulfide removal, compared with the dosing of ferric chloride solution.

#### *3.4. Impact of Temperature and Storage of Waterworks Sludge*

Out of 13 surveyed plants, 1 had experienced a slight loss of efficiency resulting from waterworks sludge dosing in the summer. When stored for a long time in the summer, the waterworks sludge lost some of its function, since iron crystals were formed on the sludge surface when it dried in the sun. The iron in the crystals was less available for the microorganisms in the digester. Only one of the producers surveyed identified this as problematic. The storage of sludge in a shaded environment and the modest addition of moisture to the sludge could mediate this issue.

#### *3.5. Observed Operational Conditions When Mixing Waterworks Sludge with Substrate and Feeding the Mix into Digesters*

The mixing and feeding of waterworks sludge into the digester was generally a carefree process. Very few problems have been experienced in connection with the handling of the waterworks sludge in biogas plants. The exception was for biogas plants utilizing solid substrates. In these plants, the substrate is mixed with waterworks sludge and fed into the digestion chamber with a screw. The screw is designed for handling dry materials. If the substrate becomes too wet after mixing with the sludge, it slides backwards and stops following the screw. With less feed into the digester, the production decreases. The solution to the problem is to mix the sludge with drier materials and preferably also with longer straw in the substrate. In one of these plants, it was observed that some of the waterworks sludge remained at the bottom of the digestion chamber when it was opened. No action

has been taken, but this suggests that the dissolution of the sludge is slow when it is fed together with solid material.

In most of the plants that use liquid substrates, the waterworks sludge is scooped into a mixing well. In these wells the pH is often quite low and the stirring is vigorous. A low pH facilitates the dissolution of the sludge as the solubility of the iron increases with decreasing pH. Those who use this type of mixture have not experienced any problems with dissolving the sludge, nor have they seen any residues of undissolved sludge in the mixing well when it has been drained. For wastewater treatment plants, operators have expressed concerns that the addition of waterworks sludge could affect the drainage properties of the digestate. The opposite effect is indicated by literature data, finding that the drainage of biosolids after the addition of waterworks sludge is improved [13,18].

#### *3.6. Transportation of Waterworks Sludge into the Digestion Plant*

The waterworks sludge is slightly adhesive and may ge<sup>t</sup> stuck on the flatbed when transported. For this reason, various measures have been taken to make handling and cleaning easier. Many people have added straw or sawdust to the flatbed before loading the sludge to make cleaning easier. Another possibility is to spray the flatbed with rapeseed oil or similar prior to loading. There is also a risk in cold climates that the sludge gets stuck on the platform due to freezing. In wintertime, it may be necessary to transport the sludge in closed containers.
