Optimering och utvärdering av avloppsslambehandling för minskad klimatpåverkan

Sammanfattning

The study examined methane formation in a laboratory setting over a 7-week period using mesophilically digested and dewatered sewage sludge from Kungsängsverket, treated with urea (0.2–1.2%) and/or lime pellets (26%) under two temperature conditions (room temperature, approx. 22°C and 30°C). The project was funded by and carried out on behalf of Uppsala Vatten och Avfall AB (UVA) with the aim of reducing the company's climate impact from the intermediate storage of sewage sludge at Hovgården. Previous studies have shown that urea addition can inhibit methane formation in sewage sludge through elevated ammonia concentrations, as ammonia toxicity inhibits methanogenic microorganisms. In addition to the laboratory component, a climate calculation was performed assessing environmental impact from a life cycle perspective, along with a cost calculation for each treatment method. The laboratory results showed that methane formation did decrease, but not linearly with urea addition, and that a 0.6% urea dose yielded approximately 5 mlCH₄/gVS, which is significantly different from the 0.4% urea addition but not significantly different from higher urea doses. The control produced approximately 30 mlCH₄/gVS. The addition of lime pellets resulted in a significant reduction in methane formation compared to the control at room temperature (approx. 22°C) but showed no methane-inhibiting effect at 30°C or when combined with urea. The climate calculations showed that the treatment options of 0.6% urea and 0.6% urea combined with lime pellets achieved a 70–72% reduction in climate impact compared to the control, alongside higher urea additions (0.8–1.2%). The cost calculations indicated that a saving of 4 million per year (SEK) is achievable through the addition of lime pellets to the sewage sludge, owing to the high cost of the current handling practices. The overall assessment concluded that 0.6% urea addition in combination with lime pellets represents the optimal treatment option. For this to be practically implemented, further assessments and dialogue with the parties involved will be required.