Walnut Shell Media Still Beats Sand in Oily Water: Where the Filter Earns Its Keep

Sand filters fail in oily water in a predictable way. Oil coats the grains, the bed binds into a hard mass, head loss climbs, and the operator is backwashing twice a shift to keep flow up. Filtration precision drops as the bed channels, and eventually the media is dug out and replaced. Plants then compensate with more chemicals upstream, which makes the sludge problem worse without fixing the filter.

The SINOKLE walnut-shell filter takes a different route: naturally modified walnut-shell media, a deep-bed graded structure, enhanced regenerative backwashing, anti-clogging water distribution and intelligent control. It is used on oilfield produced water, refinery oily wastewater, chemical and steel-mill effluent, circulating cooling water and reclaimed-water reuse, usually as the unit that guarantees the final number.

Oleophilic media: oil sticks, then lets go

The media is natural walnut shell, specially activated and modified, with a porous honeycomb structure and a specific surface area of 800–1200 m2/g. It is strongly oleophilic and hydrophobic, so it captures oil through physical interception and chemical adsorption working together, holding dispersed oil, emulsified oil and fine suspended solids above 5 µm. Oil-removal efficiency on oily wastewater runs 92%–97%, with effluent oil at ≤5 mg/L and suspended solids at ≤3 mg/L — well beyond what a sand filter of the same size will hold.

The important property is reversibility. Walnut shell is tough and elastic: it compacts into a precise filter layer during filtration and rebounds to a fluffy state during backwash, without hardening, caking or oil sticking. Oil adsorbs firmly but strips off when the bed is disturbed. With the dedicated regeneration process, media regeneration efficiency exceeds 95% and there is no performance decay over long running. Media life is 5–8 years, more than twice ordinary sand.

Deep-bed grading and long cycles

The bed is graded naturally coarse at the top and fine at the bottom, so water flowing downward meets coarse filtration at the surface, deep filtration in the middle and fine filtration in the inner layer. Pollutants distribute through the whole depth instead of plugging the top centimetre. Dirt-holding capacity reaches two to three times that of a conventional sand filter and a single filtration cycle runs 12–24 hours, which is the number that determines how often the plant is interrupted.

Backwashing that actually cleans

Combined air-water backwashing plus mechanical-disturbance regeneration does three things at once: air-flow shearing, water-flow scouring and media friction. Oil, suspended solids and other foulants are stripped off the media surface thoroughly rather than being pushed deeper into the bed. A backwash takes 5–10 min and uses only 3%–5% of the produced water — over 40% less than a conventional filter. With no high-power rotating parts, energy consumption stays low, and long filtration cycles mean the plant is not paying for backwash water or backwash power every few hours.

Footprint, materials and control

The vessel is a vertical cylinder with patented anti-clogging labyrinth water distribution and collection, no complex redundant internals, and a footprint of only one-half to two-thirds of an equivalent sand filter, with no large civil works. Standardised skid-mounted delivery means assembly, commissioning and media loading are finished before shipment; on site it is pipe and power connections, cutting installation time by more than 60%. Capacities run from 5 to 200 m3/h, which covers side-stream duty through to full-flow produced water.

Materials are chosen for corrosive industrial water: anti-corrosion carbon steel with rubber lining or epoxy coating, or 304/316L stainless steel, resisting acids, alkalis, salts and organic solvents. Internal distributors and collectors are stainless steel or high-strength ABS, so they do not deform or fail in service. Overall equipment life can exceed fifteen years.

Control is a PLC-based fully automatic system with online pressure, flow and water-quality instruments monitoring inlet and outlet pressure, differential pressure, flow, turbidity and oil content in real time. Filtration, backwash, forward wash and drain run automatically with no manual attendance, remote monitoring and parameter adjustment are available, and over-limit differential pressure, fault alarm and automatic shutdown protection are built in. There are no wearing parts, so routine work is periodic differential-pressure checks and simple maintenance — which is what keeps O&M cost low.

Specifying it correctly

Three questions decide whether a walnut-shell unit performs or disappoints. The first is what the oil actually is: the media is at its best on dispersed and emulsified oil, and a stream dominated by free oil should be skimmed or floated first rather than thrown at a filter. The second is suspended-solids loading, because dirt-holding capacity, not filtration precision, sets the cycle length in most plants. The third is backwash water disposal — at 3%–5% per backwash the volume is small, but it still has to go somewhere, and on an offshore or water-scarce site that routing is worth designing before the vessel arrives.

Get those three right and the filter tends to disappear from the operator's attention list, which is the real test of a polishing unit. Get them wrong and no amount of media quality will compensate.

Where it is installed

Used alone, the filter handles deep oil and turbidity removal from industrial oily wastewater, pre-treatment ahead of reuse, and side-stream filtration of circulating water. In series it is commonly paired with hydrocyclones, coalescers, dissolved-air flotation units and precision filters to form a complete treatment system. It has been applied across petrochemical, refining, oilfield, chemical, steel and metallurgy sites with stable long-term compliance.

The practical summary: high filtration precision, large dirt-holding capacity, long cycles, water-saving low-consumption backwash, compact skid mounting and long-life media. For plants that have accepted frequent backwashing as the cost of filtering oily water, the walnut-shell route is usually worth a serious look.