Walnut Shell Filter Media (often referred to in the industry as walnut shell sand) is a natural, chemically inert, and highly effective filtration material widely used in water treatment. It serves as a premium alternative to traditional quartz sand, especially for challenging industrial and oily wastewater applications.
Here is a comprehensive breakdown of why and how walnut shell sand is used in water treatment:
1. Core Applications (Where it is used)
- Oilfield Produced Water & Injection Water (Primary Use): In oil and gas extraction, massive amounts of oily wastewater are generated. Walnut shell filters efficiently remove micron-sized oil droplets and suspended solids, cleaning the water so it can be safely reinjected into the oil reservoir [tradeindia.com].
- Refinery & Petrochemical Wastewater: Used as a pre-treatment stage before biological treatment to protect delicate bacterial systems from being coated and killed by heavy hydrocarbons and oils.
- Steel Mill & Metallurgical Wastewater: Cleans recycling cooling water from hot rolling and continuous casting processes, trapping iron scale, grease, and suspended solids to prevent pipeline clogging.
- Bilge Water Treatment: Helps marine vessels meet strict international discharge regulations by removing emulsified and free oil from shipboard waste streams.
2. Walnut Shell Sand vs. Quartz Sand (Why choose Walnut Shell?)
When handling oily water, traditional quartz sand fails because it is oleophilic (oil-loving). Oil permanently sticks to quartz grains, causing the sand bed to “clump” or “mud-ball,” making backwashing impossible. Walnut shell sand solves this through unique properties:
- Hydrophilic & Oleophobic Surface: Walnut shells naturally attract water and repel oil. Oil droplets are mechanically trapped in the gaps between the shells rather than sticking to the material itself.
- Effortless Backwashing: Because oil does not adhere to the surface, a simple backwash cycle with water fluidization and agitation easily flushes out the trapped oil and solids, restoring the filter bed to like-new condition.
- High Attrition & Chemical Resistance: Made from high-density, tough lignocellulose, walnut shells resist crushing, rotting, and degradation under constant high-pressure backwashing. They are highly stable in both acidic and alkaline environments.
- Superior Dirt-Holding Capacity: The angular, irregular shape of crushed walnut shells creates a high-porosity filter bed. Its suspended solids (SS) capturing capacity is typically more than double that of conventional quartz sand.
3. Typical Technical Performance Indicators
| Parameter | Performance Range |
|---|---|
| Oil Removal Efficiency | 85% – 95%+ |
| Suspended Solids (SS) Removal | 85% – 98%+ |
| Common Mesh Sizes | 12/20 mesh (0.8–1.6mm), 20/40 mesh (0.4–0.8mm) |
| Filtration Velocity | 15 – 25 m/h (Allowing for compact equipment design) |
