Sewage sludge drying – efficient, low-emission, and regulatory future-proof

From January 1, 2029, the land-based application of sewage sludge is prohibited for sewage treatment plants with 100,000 population equivalents (PE) or more – from 2032, this will also apply to plants with 50,000 PE or more. Sewage treatment plants under 50,000 PE may continue to apply sludge to agricultural land.

In parallel, an independent obligation for phosphorus recovery is taking effect: Sewage treatment plants with 100,000 PE or more must recover phosphorus from 2029, and plants with 50,000 PE or more from 2032. The phosphorus obligation also applies to new plants with 10,000 PE or more from 2029. Drying the sewage sludge is a central step in this process: it is a prerequisite for thermal utilization, from whose incineration ash the phosphorus can be recovered. For many operators, thermal utilization will be the central disposal route in the future – even if the law formally allows other recovery processes.

It is not possible to make a general statement without specific project data—plant size, throughput, available heat source, and the desired dry matter (DM) content are the key parameters. As a general guideline: The thermal treatment of sewage sludge typically costs operators between €370 and €470 per metric ton of dry matter today—costs that can be partially offset by operating an in-house drying plant or reduced through negotiations with incineration facilities, since dried sludge has a significantly higher calorific value and commands more favorable terms. In addition, transportation costs are reduced due to the significantly lower weight after drying. As part of its project consulting services, STELA prepares a customized economic feasibility analysis—based on the actual sludge volume, the available waste heat, and local disposal costs.

Thanks to its modular design, the STELA BTS can be configured for wastewater treatment plants of various sizes—from smaller municipal facilities to large industrial operators. An important prerequisite: The sewage sludge must be digested, meaning it must have undergone an anaerobic digestion process prior to drying. Digested sludge no longer emits significant amounts of gas, which greatly simplifies explosion protection.
The initial dry matter (DM) content depends on the feed system: Drying via recirculation is possible starting at approximately 10% DM; after mechanical dewatering, the dry matter content typically ranges from 20 to 30% and is then fed via a pump with a matrix (“STELA Matrix Feed”). Whether a dedicated plant or a shared solution with neighboring wastewater treatment plants is more economical depends on the sludge volume and the local infrastructure. STELA advises operators as early as this planning phase.

The target dry matter content depends directly on the planned utilization route: For co-incineration in hard coal or cement power plants, a dry matter content of at least 60 to 70% is generally required. For mono-incineration in fluidized bed furnaces – the preferred process in Germany, which enables targeted phosphorus recovery from the ash – dry matter contents of 80 to 95% are typically targeted. For pyrolysis, specific requirements apply depending on the plant manufacturer. Co-incineration in coal-fired power plants is limited in the medium term as a permanent utilization route. STELA designs the drying plant for the agreed target dry matter content and ensures compliance through an automatic moisture measurement control system.

The BTS can be adapted in a modular fashion to different slurry consistencies: Liquid sludge with a dry matter (DM) content as low as approximately 10% can be processed via recirculation; pumpable and moldable sludge with a DM content of approximately 20% or higher can be processed via a pump with a matrix (“STELA Matrix Feed”); and solid and lumpy sludge with a DM content of approximately 40% or higher can be processed via a double-screw distributor. The dry matter (DM) content indicates what percentage of the sludge is actually solids; the rest is water. The target moisture content after drying can be individually adjusted depending on the recovery method. For thermal recovery via incineration or pyrolysis, dry matter contents of 80 to 95% are typically targeted.
 

Low-temperature belt dryers like the STELA BTS operate with process temperatures below 120 °C. This offers two significant advantages over high-temperature processes: emissions are significantly lower, and the explosion risk from volatile gases in the drying process is considerably reduced. Drum dryers require significantly higher temperatures, which increases energy consumption and makes exhaust air treatment technically more complex and costly.

Since the BTS operates with low temperatures and minimized exhaust air volumes, the necessary exhaust air treatment is smaller from the outset than with high-temperature processes. Depending on site requirements and permit conditions, multi-stage exhaust air scrubbers, biofilters, or combination solutions are used. The significantly reduced exhaust air volume lowers both the investment costs and the ongoing operating expenses of exhaust air purification.

STELA Laxhuber assumes to dry only digested sewage sludge. Digested sludge no longer emits any significant gases, so explosion protection focuses on dust emissions. The BTS is therefore designed and manufactured according to a comprehensive explosion protection concept that meets the requirements for dust explosion protection. The dryer as a whole is not certified separately; the explosion risks are assessed as part of the CE risk assessment and addressed by the protection concept.

Yes. Industrial sludges from the paper industry, food processing, or chemical industry can have widely varying pH values, solid compositions, and pollutant contents. These properties influence the choice of materials, the design of the process belts, and the necessary exhaust air treatment. STELA conducts individual drying tests for every application and designs the plant specifically for the specific sludge properties.

The modular construction of the BTS allows for a precise design tailored to the actual throughput requirement – from compact individual plants for smaller wastewater treatment plants to multi-stage systems with high capacity for large industrial operators. A subsequent capacity expansion is generally possible, which increases investment security and allows for gradual scaling.

Phosphorus is a finite raw material that is indispensable for agriculture. During the thermal utilization of dried sewage sludge, phosphorus concentrates in the incineration ash and can be specifically recovered from there. Drying is therefore a technical prerequisite for closed phosphorus cycles. In Germany, the Sewage Sludge Ordinance (AbfKlärV) obliges operators of larger wastewater treatment plants to recover phosphorus from 2029 or 2032 onwards – investing in a drying plant is therefore also a regulatorily future-proof decision.