Oily, detergent-laced sludge from equipment washing can hold >90% water and tip into hazardous territory. Simple drying beds and geotextile bags are cutting volumes fast—if pretreatment and B3 (hazardous) rules are respected.
Equipment-wash stations at coal mines generate oily sludge loaded with sediment, industrial detergent, and petroleum residues. An Indonesian study of heavy‑equipment washing described the material as a mixture of soil, water, detergents, and lubricants (researchgate.net). The sludge often retains high moisture (often >90%) and can concentrate hydrocarbons and trace metals from engine oils and brakes.
Disposal hinges on what’s inside the cake. If metal or hydrocarbon concentrations exceed regulatory thresholds, the dried sludge is classified as hazardous (B3) waste under Indonesian law (comparable to used oil and grease) and must be tightly controlled (ppid.menlhk.go.id). Mines typically tackle free oil first with oil–water separators or gravity skimmers to reduce the hydrocarbon load before dewatering (researchgate.net), a sequence akin to the screens, oil removal, and primary units bundled under waste-water physical separation.
Drying beds: passive sand-and-sun dewatering
Open drying beds are a low‑cost, passive method: sludge is spread 20–30 cm deep over sand and gravel layers and left to drain and evaporate (waterandwastewater.com). Design guidelines suggest loading on the order of 100–250 kg of dry solids (DS) per m²·yr (kilograms of dry solids per square meter per year), roughly 0.5–1.2 m³ of wet sludge, to avoid ponding (waterandwastewater.com).
In warm, dry climates, beds often yield a semi‑solid cake with moisture <70–80%. A U.S. EPA sludge manual notes that in fair weather a small polymer dose (~0.5 lb per dry ton) can cut drying time from ~13 days to ~5 days (nepis.epa.gov). Sand‑bed systems can capture >90% of solids and produce drained effluent amenable to simple recycling (gssb.com.my; nepis.epa.gov). Maintenance involves periodically scraping out the dried cake (often 4–6 weeks in humid climates, shorter in arid zones) and replenishing media.
Geotextile bags: portable, modular dewatering
Geotextile “bags” or tubes (woven polypropylene sacks) offer a portable, modular option. Sludge is pumped in; water passes through the fabric while solids form a cake inside. Filtration efficiency can be very high (often ≳90% solids retained) because a filter cake quickly builds up in the fabric (gssb.com.my).
Drainage is slower—typically several days. One report notes geotubes draining about 70% of water in ~7 days (gssb.com.my). Dried sludge inside consolidates (volume reductions of perhaps 50–80% over several days are typical), with bag size chosen to match projected sludge volume and site footprint. The method avoids large‑area leveling but requires eventual handling of dried cakes, similar to drying beds.
Pretreatment and conditioning steps
Small oil/water separators (API skimmers; compact oil–water devices) and settling pits remove free oil and heavy sediment before dewatering. For the oil phase, mines lean on separation equipment such as oil removal systems designed to bring free oil to <10 ppm, aligning with the skimming-first approach detailed above (researchgate.net).
Coagulation/flocculation ahead of geobags or sand beds is common: adding cationic polymer (e.g., 0.2–1 kg per tonne of sludge) significantly improves drainage and final cake solids (nepis.epa.gov). Mines use precise chemical feeds via a dosing pump and choose from flocculants to optimize performance. In practice, a train of treatments is used: first coarse settling/grit removal, then oil‑skimming, then dewatering (bed or geotube) to maximize water recapture.
Volume reduction and design metrics
Field data show dramatic volume reduction: passive solar drying beds in sewage treatment commonly produce an 80–90% shrinkage (by volume) of raw sludge during drying. In mining wash bays, moisture removal is aided by gritty solids (grit hastens draining) and strong sunlight in Indonesia.
Key design metrics: if a mine produces 5 mgd (a flow‑rate unit common in water engineering; ≈5700 L/min) of washwater with ~500 mg/L TSS (total suspended solids), that is ~86 kg TSS/hr. At 250 kg DS/m²·yr loading, one m² of drying bed could handle roughly 10 kg DS/day, so ~8.6 m² per 86 kg. In other words, each m² of bed reduces ~10 kg dry solids yearly, or ~5–10 m³ of sludge per year. These are rough engineering guidelines derived from municipal protocols (waterandwastewater.com), but they illustrate that modest bed areas can handle significant sludge if drained over weeks.
In well‑dried conditions, final cake solids often reach 20–30% (w/w, meaning weight fraction), so >70% of the weight is removed as water/effluent.
B3 classification and disposal routes
Indonesian regulations (PP101/2014 and follow‑ons) require all hazardous (B3) waste to be treated or disposed in controlled facilities: generators must use licensed B3 transporters and send it to a certified B3 landfill or incinerator (ppid.menlhk.go.id). Used oil from mining is designated category B105d, necessitating managed disposal. In practice, mines often contract specialist waste firms for oily sludge removal, ensuring strict chain‑of‑custody and final incineration or stabilization in engineered B3 landfills (ppid.menlhk.go.id).
Reuse, circular options, and water recapture
Where possible, recycled uses are pursued: the Indonesian environment agency encourages “circular” reuse of mining residues (ppid.menlhk.go.id). High‑carbon sludge with low metals could be co‑processed (e.g., burned as fuel adjunct). Used lubricants are sometimes repurposed as fuel in explosives (ANFO) for blasting (ppid.menlhk.go.id), and coal tailings have been used in paving blocks or road base. One mining company reported saving ~27.5 billion IDR (~USD1.9 million) in 2019 by recycling used oil and grease (cutting oil use by ~1000 tonnes) (ppid.menlhk.go.id).
Even where equipment‑wash sludge is dirtier, clean fractions can be diverted: clarified effluent from dewatering (typically >90% of the volume) can often be reused for wash water (researchgate.net), saving fresh water and reducing net waste.
Figures, outcomes, and scheduling
Simple dewatering steps can shrink wash‑bay sludge volumes by roughly an order of magnitude. Geotube systems see effluent loss of ~70–90% (implying volume reduction >70%) (gssb.com.my; gssb.com.my). Drying‑bed solids loading guidelines (100–250 kg DS/m²·yr) mean a 1000 m² bed might process 100–250 tonnes of dry sludge annually (waterandwastewater.com).
Each 1 tonne of sludge contains hundreds of kilograms of water; a single dewatering cycle will often remove hundreds of litres of water per m² of bed. Operator case studies show that adding minimal polymer (0.5 lb/ton DS) cuts drying time by >60% (nepis.epa.gov), meaning schedules and equipment footprints can be significantly optimized.
Bottom line: sequence, size, and certify
For mining operations, gravity separation followed by either sand beds or geotextile bags can vastly reduce sludge volume and water content, lowering disposal costs and freeing up water for reuse. All treated sludge must be tested: if heavy metal/hydrocarbon tests exceed limits (per Indonesian B3 thresholds), it must be sent to authorized hazardous waste facilities (ppid.menlhk.go.id). Otherwise, even dewatered “non‑B3” sludge requires containment.
The data‑driven recommendation: install pretreatment (settling + oil trap), then dewatering beds or geobags sized per design loads (100–250 kg DS/m²·yr) (waterandwastewater.com), monitor sludge hydrocarbon/metal levels, and dispose of cakes at licensed B3 facilities if needed—or pursue approved recycling under circular‑economy initiatives (ppid.menlhk.go.id). From a business‑case perspective, large Indonesian coal mines report savings in the multi‑million‑dollar range by recycling oil and water rather than disposing them (ppid.menlhk.go.id; researchgate.net).
Sources and design manuals: drying beds and loading rates (waterandwastewater.com); U.S. EPA sludge manual with polymer guidance (nepis.epa.gov); Indonesian regulatory guidance and industry reports (ppid.menlhk.go.id; researchgate.net); geotextile dewatering performance (gssb.com.my; gssb.com.my).
