A containerized treatment train is quietly solving one of midstream’s dirtiest secrets: hydrotest water loaded with rust, oil, and organics. The playbook is filtration first, then carbon—validated by lab tests before and after the job.
Hydrostatic (pressure) testing—filling a pipeline with water and pressurizing it—moves astonishing volumes. One documented field case ran a mobile skid at 1,000 gal/min (≈ 227 m^3/h) to process about 3.35 million gallons (~12.7 million liters) of hydrotest water (www.evoqua.com). That water doesn’t come back clean. It can pick up rust, scale, sediment, oils/hydrocarbons, and chemical residues introduced during pigging, cleaning, or prior service (www.xylem.com) (www.xylem.com).
The stakes are regulatory and reputational. In Indonesia, for example, effluent from oil & gas activities must meet strict limits—oil & grease ≤15 mg/L and COD (chemical oxygen demand) ≤110 mg/L—under Permen LHK No.19/2010 (id.scribd.com). Meeting those numbers on a remote right‑of‑way demands a portable treatment train that strips suspended solids first, then tackles dissolved organic compounds with adsorption.
Pre‑ and post‑test water quality monitoring
Rigorous testing before and after the hydrotest is the control knob that makes the system work. Pre‑test sampling establishes a baseline—pH, conductivity/TDS (total dissolved solids), dissolved oxygen, oil & grease, and metals—so the skid can be configured correctly. Post‑test (pipeline discharge) water almost always shows elevated suspended solids, corrosion products, and possibly organics, with new or cleaned pipelines often yielding TSS (total suspended solids) <100 mg/L but up to ~1,000 mg/L if the line isn’t fully pigged; in‑service pipelines can run >1,000 mg/L and even approach ~2,300 mg/L in extreme cases (www.scribd.com) (www.scribd.com).
Oil and grease is usually modest (<50 mg/L) in new‑pipe tests, but liquid‑product lines can carry significant hydrocarbons—BTEX (benzene, toluene, ethylbenzene, xylene), phenols, and TPH (total petroleum hydrocarbons)—as product “carry‑over,” with two orders of magnitude variability observed (www.scribd.com) (www.scribd.com). Metals can spike too; one offshore test saw water turn bright rust‑red on exposure, signaling dissolved iron (pgjonline.com).
Guidelines recommend analyses of pH, TSS, turbidity, TDS/EC (electrical conductivity), temperature, oil & grease, COD/BOD (biochemical oxygen demand), specific toxins (BTEX/phenols), heavy metals, and residual disinfectants for discharge monitoring (www.scribd.com). A treatment permit might specify limits of TSS and oil/grease in the single‑digit mg/L range, so post‑treatment sampling must confirm compliance (www.scribd.com) (id.scribd.com). Practically, if the influent has 800 mg/L TSS, testing will drive the addition of a rapid‑settling clarifier or filter train before adsorption.
Contaminants and removal targets
Suspended solids—rust, sand, pig debris—dominate. For new‑pipe tests, “typically <100 mg/L but as high as 966 mg/L” are reported; in fouled, in‑service lines the number can hit 2,348 mg/L (www.scribd.com) (www.scribd.com). A robust system is designed to remove ≥95–99% of TSS, targeting <5 mg/L in the effluent.
Oil/grease and dissolved organics vary. New‑pipeline tests usually see oil & grease <50 mg/L (www.scribd.com), but product carry‑over can introduce BTEX, phenols, and TPH at widely varying levels (www.scribd.com). That is why mobile skids pair oil/water separation for free and emulsified oil with granular activated carbon (GAC) to remove dissolved organics—benzene, phenols, and more—achieving roughly 90–99% removal of BTEX/TPH under proper conditions (www.xylem.com) (www.evoqua.com).
Dissolved metals and chemicals are addressed with pH control and oxidation where needed; neutral/alkaline conditions reduce iron solubility. If source water is chlorinated, residual chlorine is removed to avoid byproducts. Microbial content is usually not a primary concern unless the water is destined for potable use; for reuse polishing, UV (ultraviolet) or chlorination is often included.
Typical pH ranges ~6.5–8.5 for hydrostatic test waters, and final effluent generally must land within ~6–9 per permit (www.scribd.com) (www.scribd.com). Conductivity/TDS typically stay steady unless salt was used; very high‑TDS feeds may need added steps, which pre‑test analysis will flag.
Mobile treatment train architecture
Screening and grit removal come first. A coarse screen—akin to a bar screen—keeps wood, pig debris, and large solids out. For this duty, many field packages include a compact intake such as an automatic screen to continuously remove debris >1 mm while minimizing manual handling.
Settling is next for heavy solids. Mobile units often deploy sedimentation or compact plate settlers; a lamella module such as a lamella settler reduces footprint compared with conventional basins while dropping the bulk solids load ahead of filtration. Where space allows, a conventional clarifier provides a familiar gravity step.
For very turbid water, coagulation/flocculation is added. A metered chemical feed—delivered by a dosing pump—injects alum or polymer to clump fines, followed by flash mix and gentle flocculation. Field teams often keep coagulants and floc aids on the skid to rapidly drive turbidity down before filtration.
Media filtration is the workhorse for suspended solids. Multi‑media beds using sand/silica media progressively remove particles to single‑digit mg/L. Dual‑media designs commonly include an upper layer of anthracite to increase dirt‑holding capacity and extend run times.
To catch fines and surges, crews add cartridge trains. Pleated elements in the 2–4″ class routinely capture down to 5–10 µm; that’s where a robust cartridge filter stage earns its keep, often housed in high‑pressure vessels such as a steel filter housing to handle industrial flows.
Oil/grease removal follows in hydrocarbon‑exposed lines. Gravity or coalescing separation removes free and emulsified oil, complemented by an oil removal module to polish down to low mg/L. For sub‑10 mg/L targets, a downstream carbon stage adsorbs residual droplets and dissolved fractions.
Activated carbon adsorption is the dissolved organics insurance policy. GAC vessels are sized for 5–15 minutes of EBCT (empty bed contact time). At 1,000 gpm (~227 m^3/h), a ~10‑minute EBCT equates to roughly 38 m^3 of carbon across multiple pressure vessels; in practice, mobile systems string several beds in series/parallel. With adequate contact time, GAC reduces benzene, toluene, phenol, and similar constituents by >90%, often to non‑detectable levels. Spent carbon is changed out and disposed of as hazardous waste per regulations. For media supply and change‑outs, teams standardize on activated carbon designed for organics removal in industrial waters.
Final polishing steps depend on the discharge or reuse plan. A microfilter (0.5–1 µm) or UV disinfection can neutralize microbial risks ahead of reuse. If prior chlorination was used, a dechlorination agent knocks residual chlorine down to protect receiving waters and downstream materials.
Sizing, throughput, and run‑time metrics
The 1,000 gpm case treated ~3.35 million gallons in a continuous run of ~60 hours (www.evoqua.com). To sustain that, pumps, headers, and filters are sized for the full flow, often via multiple 48–60″ filter housings or multi‑vessel banks in parallel. Designers hold filter velocity under ~5 gpm/ft² on rapid sand filters to keep headloss and breakthrough in check, and often deploy staged cartridges to guard the carbon.
Performance targets center on solids and oil: TSS <5 mg/L, O&G <5–10 mg/L, and organics below detection or below permit limits (for context, Indonesia “drainase” limits for oil are ~15 mg/L; aiming below provides margin) (id.scribd.com). A deep bed sand filter rated for 50–100 mg/L influent often delivers a few mg/L in the effluent, with cartridges mopping up fines.
Activated carbon design follows influent loading and contact time. A ~10‑minute EBCT at 1,000 gpm calls for ~38 m^3 of GAC; typical carbon capacities (~0.5–1.0 kg organics per kg carbon) mean that processing ~12 million liters with, for example, 50 mg/L diesel‑range TPH can consume several tons of carbon over a campaign.
Why testing before and after matters
Pre‑test sampling prevents corrosion and compliance surprises. Codes such as API/ASME stress using low‑chloride water (<50 mg/L Cl⁻) for stainless and carbon steel tests (www.linkedin.com). If the source water is high in chlorides or acidic, pre‑treatment or chemical correction is needed. Baseline testing also surfaces unusual pollutants before the line is filled.
Post‑treatment sampling provides the proof. Measurements of TSS, oil, COD, BTEX, metals, and pH after treatment confirm compliance. In one crude pipeline hydrotest, the deployed mobile system met the discharge permit—removing benzene, total petroleum hydrocarbons, TSS, and more (www.xylem.com) (www.xylem.com). In Indonesia, any discharge must meet the Permen LHK No.19/2010 limits—oil & grease ≤15 mg/L and COD ≤110 mg/L—so those post‑treatment tests serve as final proof (id.scribd.com).
Data‑driven outcomes and solids math
Empirically, mobile packages deliver ≥90% removal of TSS and organics. As a simple benchmark: influent at 200 mg/L TSS and 20 mg/L oil can be polished to <10 mg/L TSS and <2 mg/L oil via a filter‑plus‑carbon train. With proper contact time, carbon beds routinely exceed 90% removal for BTEX/TPH.
Designs also plan for worst cases. Canadian guidelines put influent TSS “usually <100 mg/L but sometimes ~1,000–2,300 mg/L” (www.scribd.com) (www.scribd.com). If a designer assumes 2,000 mg/L and targets <5 mg/L in the effluent, the solids to remove are 1.995 g/L. At 227 m^3/h (the 1,000 gpm case), that’s roughly 452 kg of solids captured per hour—an argument for robust settling plus deep‑bed filtration and staged cartridges.
When water is scarce, recycle becomes part of the plan. With real‑time monitoring (turbidity meters) and grab sampling after each polishing pass, operators commonly recycle 80–90% of test water through the skid. Those data‑driven controls help crews hit schedule, with the 1,000 gpm system cited earlier treating ~3.35 million gallons in ~60 hours and returning the pipeline to service on time (www.evoqua.com).
Sources and field references
Xylem details how hydrotest water “can become contaminated with rust, suspended solids, chemical impurities,” and describes mobile filtration/carbon systems designed to meet permits (www.xylem.com) (www.xylem.com). Evoqua documents a deployed system that treated ~3.35 million gallons at 1,000 gpm (www.evoqua.com). Pipeline engineers have reported rust‑coloring from dissolved iron when water is left untreated (pgjonline.com). Canadian hydrotest guidelines provide the TSS and oil ranges cited above (www.scribd.com) (www.scribd.com) (www.scribd.com). Indonesian regulation Permen LHK 19/2010 sets effluent limits such as oil & grease ≤15 mg/L for “drainase” waste and COD ≤110 mg/L (id.scribd.com).
