15–35% solids (15–35% TS)
Also known as: dewatered digestate solids · separator solid fraction solids
The typical dry solids content of the solid fraction from a mechanical digestate separator — high enough to be stackable and transportable as a semi-solid organic fertilizer.
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What is 15–35% solids?
A solids content of 15–35% characterises the dry-matter range of the solid fraction produced by mechanical separation of biogas digestate. This range is the typical output of screw presses and decanter centrifuges operating on the liquid digestate stream — raw digestate enters at 4–10% dry matter and the separator concentrates it into a stackable cake of 15–35% DM, while the liquid fraction exits at 4–8% DM.
Achieving 15–35% solids transforms the practical handling characteristics of digestate. Below 15% DM, the material is semi-liquid — it flows under gravity, requires tankers for transport, and cannot be stored in heaps without losing nutrients to leachate. Between 15% and 25% DM, the cake is plastic and friable, can be loaded by front-end loader, and can be heaped 2–3 m high. Above 25% DM, the material behaves like wet compost — it can be conveyed by belt, fed into pelletising lines, and bagged for retail sale. The dry-matter level achieved depends on the separator type: a screw press typically delivers 18–28% DM, while a decanter centrifuge with polymer addition reaches 25–35% DM.
Trade-offs across the range have direct economic consequences. Higher DM means lower water mass transported and stored, which cuts logistics cost dramatically — moving 1 tonne of 25% DM cake delivers as much nutrient as 1.7 tonnes of 15% DM cake. However, getting from 18% to 28% DM requires either polymer flocculant addition (₹50–80 per m³ processed) or upgraded equipment (decanter instead of screw press). The optimisation point depends on logistics radius: plants serving customers within 30 km can operate cost-effectively at 18–22% DM, while plants serving 100+ km radius need 28–35% DM to make freight economics work. Many SATAT plants now design separation in two stages — primary screw press for bulk dewatering followed by composting or thermal drying to reach 50%+ DM for retail-grade pelletised fertiliser. Throughout, the 15–35% range represents the natural dry-matter window where digestate transitions from waste-management liability to fertiliser product.
Common questions about 15–35% solids
Plain-English answers to what people most often ask.
Is a centrifuge better than a screw press for higher solids?
Can I sell solid digestate at 15–35% TS without further processing?
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