RO Unit for Oil and Gas Extraction Sites 20m3/day with 70% Recovery

During oil and gas extraction, the process often brings up highly brackish water from underground. This production water can vary significantly in quality, but it typically contains very high levels of Total Dissolved Solids (TDS)—ranging from 7,000 to 10,000 mg/l—which far exceed the standards set by the Central Pollution Control Board (CPCB). To ensure this water is safe for discharge and meets environmental regulations, it must be treated to reduce its TDS content. Installing a Reverse Osmosis (RO) plant is an effective solution to bring these levels down and protect our water resources.

Typical Test Results of production water.

Sr No	Parameters	Unit	Avg. result of untreated Effluent
1	pH	-	7
2	Temperature	deg C	43
3	Total Dissolved Solids (TDS)	mg/l	7200-10000
4	Total Suspended Solids (TSS)	mg/l	50-100
5	Chemical Oxygen Demand (COD)	mg/l	100-150
6	Biological Oxygen Demand (BOD)	mg/l	25-50
7	Oil & Grease	mg/l	50-250
8	Chlorides	mg/l	5000
9	Sulphide	mg/l	1-2
10	Zinc	mg/l	0.5
11	Phenolic	mg/l	BDL
12	Cyanides	mg/l	BDL
13	Fluorides	mg/l	5
14	Sulphates	mg/l	4055
15	Chromium (Total)	mg/l	0.2
16	Chromium (Hexavalent)	mg/l	BDL
17	Copper	mg/l	0.1
18	Lead	mg/l	0.1
19	Mercury	mg/l	BDL
20	Nickel	mg/l	0.4
21	% Sodium	mg/l	75

Designing RO system for Capacity 20 M3/Day (01 m3/Hr x 20 hrs operation/day) feed capacity for 70% Recovery.

• System overview and design basis

Item	Specification
Feed capacity	1 m³/h (20 h/day → 20 m³/day)
Recovery target	70% overall (permeate ≈ 0.7 m³/h; reject ≈ 0.3 m³/h)
Feed temperature	43°C (cooling recommended to ≤38°C for stability)
Key feed concerns	O&G 50–250 mg/L, TSS 50–100 mg/L, sulphate 4,055 mg/L, chloride 5,000 mg/L, TDS 7,200–10,000 mg/L
Membrane class	Brackish RO elements with high sulfate tolerance and high-temp rating
Array concept	2-stage RO: Stage-1 ≈ 40–45% recovery; Stage-2 ≈ 25–30% to reach ~70%
Materials	FRP pressure vessels; better for high-chloride service; EPDM/FKM seals

•  Challenges and solutions  for 70% recovery

Challenge	Risk at given feed	Engineering solution	Why it works
High oil & grease (50–250 mg/L)	Membrane fouling and wetting	CPI/API separator or induced gas flotation (DAF), followed by nutshell/carbon filters; target <5 mg/L to RO	Removes free/emulsified oil to protect RO; used in petroleum wastewater RO trains
TSS 50–100 mg/L, colloids	Particulate/organic fouling	Coagulation-flocculation + dual media filtration; 1–5 µm cartridge; optional UF for tight control	Multi-step solids removal is standard for brackish/oily feeds; UF improves RO uptime
High sulphate (4,055 mg/L) with unknown calcium	Aggressive CaSO₄ scaling at 70% recovery		Sulphate scales are there; antiscalants + pH control + staged recovery manage supersaturation
Temperature 43°C	Near membrane limit; flux/pressure shifts	Cool to ≤35–38°C	Keeps within membrane specs, stabilizes flux and reduces chemical stress
COD/BOD (100–150/25–50 mg/L)	Biofouling and organic films	Oxidant dosing (pre-RO) through UF; dechlorinate before RO; periodic alkaline clean (CIP)	Disinfection prior to RO and regular CIP control biofouling;
Chloride 5,000 mg/L	Corrosion of metals; cleaner compatibility		Materials selection avoids chloride corrosion; standard for brackish plants
70% recovery concentration polarization	Flux loss, scaling onset	High crossflow velocity; stage RO (e.g., 2-stage 40/30%); interstage antiscalant; permeate backpressure minimized	Staging distributes recovery and dosing, reducing local supersaturation
Metals (Ni 0.4 mg/L, Zn 0.5 mg/L, Cr total 0.2 mg/L)	Fouling/complexation, discharge limits	dose coagulant;	Keeps membranes clean and supports discharge compliance
Fluoride 5 mg/L, sulphide 1–2 mg/L	Materials/cleaner safety; H₂S risk	Pre-oxidize sulphide	Controls odour/toxicity and protects membranes

• Pretreatment and RO configuration

Pretreatment train

1. Oil removal:

   • CPI/API separator or DAF: Cut free/emulsified oil.

   • GAC filter: Polish to <5 mg/L oil & grease.

2. Solids control:

   • Dual media filter (MMF) to <5 NTU and SDI <3;

   • Cartridge filtration 5→1 µm;

   • Optional UF when  feed varies. Effective pretreatment reduces fouling, as emphasized for brackish waters and oily wastewater

3. Conditioning and dosing:

   • pH adjustment to 6.0–6.5 for carbonate/sulphate control;

   • Antiscalant rated for high sulphate; interstage dosing;

   • Biocide or chlorination upstream of UF/MMF; dechlorination (SMBS) before RO elements.

4. Temperature control:

   • Cooling to ≤38°C

 RO sizing:

• Twelve 4" elements (≈86–90 m² total area) sized for conservative flux 10–12 LMH at high salinity and temperature, meeting 1.0 m³/h feed and 70% recovery with margin.

• Two-stage array with interstage antiscalant/pH trim reduces local supersaturation.

Filters:

• ACF sized for EBCT 7–10 minutes at 1 m³/h to ensure dechlorination and organics polishing; MGF sized to achieve SDI ≤ 3 and turbidity < 1 NTU.

Dosing:

• NaOCl: 2–5 mg/L typical dose; adjustable 1–10 lph pump covers demand and residual control.

• SMBS: 0.1–2.0 lph metering covers typical 0–3 mg/L residual neutralization; interlocked to ORP.

• Antiscalant: 0.1–2.0 lph for fine control across 3–8 mg/L dose windows.

• Mechanical equipment list

SL. NO.	UNIT & EQUIPMENT	TYPE	CAPACITY/ SPECIFICATION	QTY.	PREFERRED MOC
1	Feed pumps	Horizontal end-suction, VFD	1.5 m³/h @ 25 mwc	2 (1W+1S)	SS316 wetted parts
2	Agitator for lime tank	Top-entering	0.75 kW; 50–60 rpm; hydrofoil impeller sized for 500 L slurry	1	SS304 shaft/impeller
3	Lime dosing pump	Diaphragm metering pump	10–40 lph @ 20 mwc (slurry capable)	2 (1W+1S)	PP/PTFE wetted end
4	NaOCl dosing tank	Cylindrical with cover	500 L	1	HDPE
5	NaOCl dosing pump	Diaphragm metering pump	1–10 lph @ 20 mwc (adjustable)	2 (1W+1S)	PP/PTFE wetted end
6	Multi grade filters (MGF) with internals	Vertical pressure filter	Service flow 1.0 m³/h; vessel ID ~600 mm; HOS 1.5 m	1	FRP (preferred)
7	Media required for MGF	Sand + anthracite	Anthracite 0.8–1.2 mm (200 kg); Sand 0.6–0.8 mm (250 kg); Gravel support (50 kg)	1 set	Sand and anthracite
8	Activated carbon filters (ACF) with internals	Vertical pressure filter	Service flow 1.0 m³/h; EBCT 7–10 min; vessel ID ~800 mm	1	FRP (preferred)
9	Media required for ACF	Sand + GAC	GAC 8×30 mesh (300 kg); Sand 0.6–0.8 mm (150 kg); Gravel support (50 kg)	1 set	Sand and carbon
10	Antiscalant dosing tank	Cylindrical with cover	200 L (with mixer port)	1	HDPE
11	Agitator for antiscalant dosing tank	Top-entering	0.37 kW; 100–150 rpm	1	SS304 shaft; PP impeller
12	Antiscalant dosing pump	Diaphragm metering pump	0.1–2.0 lph @ 20 mwc (fine control)	2 (1W+1S)	PP/PTFE wetted end
13	SMBS dosing tank	Cylindrical with cover	200 L	1	HDPE
14	Agitator for SMBS dosing tank	Top-entering	0.37 kW; 100–150 rpm	1	SS304 shaft; PP impeller
15	SMBS dosing pump	Diaphragm metering pump	0.1–2.0 lph @ 20 mwc (fine control)	2 (1W+1S)	PP/PTFE wetted end
16	Acid dosing tank	Cylindrical with cover	500 Liters	1	HDPE
17	Acid dosing pump	Diaphragm metering pump	1–10 lph @ 20 mwc	2 (1W+1S)	PP/PTFE wetted end
18	Micron cartridge filter	Multi-round housing	5-round, 20" length, rated 2 m³/h; design ΔP 1.0 bar	1	SS316
19	Cartridges for MCF	DOE/226 O-ring	5 µm (5 Nos.) + 1 µm (5 Nos.)	1 set each	PP
20	RO high pressure pump	Multistage centrifugal, VFD	1.0 m³/h @ 22 bar differential (suction 0.5–1.5 bar)	1	SS316 hydraulics ( for high chloride)
21	RO skids	2-stage skid with interstage dosing	Stage-1: 2 pressure vessels in parallel; Stage-2: 1 vessel; full-frame with drip tray	1	SS304 frame
22	RO membranes	Brackish high-rejection, high-temp rated	4" elements, total 12 Nos. (Stage-1: 2×4; Stage-2: 1×4)	12	Polyamide thin-film composite
23	RO pressure tubes	4" pressure vessels	Stage-1: 2 Nos.; Stage-2: 1 No.; 4 elements per vessel	3	FRP
24	pH correction dosing tank	Cylindrical with cover	200 L	1	HDPE
25	Agitator for pH correction dosing tank	Top-entering	0.25 kW; 100–150 rpm	1	SS304 shaft; PP impeller
26	pH correction dosing pump	Diaphragm metering pump	1–10 lph @ 20 mwc	2 (1W+1S)	PP/PTFE wetted end
27	RO CIP tank	Dual-tank (alkaline/acid)	2 × 500 L with level, drain, heater ports	1 set	FRP
28	RO CIP pump	End-suction centrifugal	5 m³/h @ 25 mwc	1	SS316
29	Cartridge filter for RO CIP	Single-round housing	1-round, 20", rated 2 m³/h	1	SS316
30	Cartridges for RO CIP cartridge filter	DOE	5 µm (2 Nos.), 1 µm (2 Nos.)	1 set	PP

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