Walnut Shell Filter

Walnut Shell Filter

Walnut Shell Filter Working Principle

The treatment of oily wastewater with filtration technology represents a new development in oily wastewater treatment methods, primarily used for separating finely dispersed emulsified oil that cannot naturally float to the surface in oil separators. The filtration method for separating finely dispersed emulsified oil in water, also known as coarse particle coalescence (Coalescence), relies on the hydrophobic coalescence of oil droplets on the filter medium surface, forming an oil film that is then washed away by the shear force of water between filter layer pores, resulting in the rise and separation of coarse oil droplets.

Based on the principles of physical-chemical flotation kinetics, the size of the cohesive force (F) exerted by finely dispersed oil droplets in water on the filtration medium depends on the diameter (r) of the oil droplets in water and the interfacial tension between the two phases of oil and water, and it is as follows: F = 4r. From this equation, it can be seen that a larger diameter (r) of oil droplets in water and a higher interfacial tension between the two phases of oil and water are favorable for coarsening. Increasing the concentration of inorganic salts in water can enhance the interfacial tension.

Walnut shell filters utilize the aforementioned principle and the porous nature and large surface area of specially treated wild walnut shells to trap suspended solids and oil droplets on the filter media surface or adsorb them. After running for a period, when the filter media becomes saturated and water quality deteriorates, the water supply is stopped for backwashing. This restores the filter media's original properties, and the backwashing water carries away the intercepted matter, achieving the effect of filtering out oil and mechanical impurities.

Equipment Structure and Features 

The complete set of equipment consists of components such as filters (in series in multiple cases), main header, oil and gas manifold, liquid and gas control systems, base, stair platform, and automatic control systems, with a carbon steel undercarriage skid.

To ensure filtered water quality, the filter employs a rational water distribution and collection system internally. It considers both the uniformity and stability of water intake and output while also simplifying the structure of the two systems to prevent complications for subsequent processes such as corrosion prevention.

The supervisor pipeline for inflow and outflow is vertically arranged in a multi-layered space, featuring a compact structure, aesthetically pleasing layout, and reduced equipment footprint.

Selecting wild walnut shells that have been specially processed as the filtration medium, the filter material boasts stable chemical properties (resistant to acid and alkali corrosion), high strength, excellent compressive resistance, and good wear resistance. Long-term use is not required, as it has strong clogging resistance, good water immersion properties, and oil resistance. It can be backwashed and regenerated, with a simple and convenient regeneration process, no need for chemicals, and the filtered water can be directly used for backwashing.

Prior to being loaded into the filter, the walnut shell filter media undergoes full pretreatment, including a single particle size screening. Particle size grading can be tailored according to the requirements for import/export water quality, offering strong specificity.

This filter utilizes walnut shell and double-layered filter media. The upper layer of walnut shell media primarily removes oil and intercepts larger suspended particles, while the lower layer of gravel media intercepts smaller suspended particles. During backwashing, due to the difference in filter media specific gravity, combined with a special water distribution technique and supplemented by an automatic switching program, it ensures that the two layers of filter media do not mix after backwashing. This achieves a reasonable division of labor in the filter layer, thereby meeting the technical requirements for filtering out suspended particles in the effluent.

Reverse washing utilizes filtered water for reverse washing, while also employing a vortex propulsion type agitator. During the entire filter's reverse washing, the water flow condition is favorable, significantly increasing the likelihood of friction and collision between the filter media. This modification from the conventional air-water soft washing to a mechanical strong washing method solves the technical challenges of filter cake, biological caking, and oil adhesion during reverse washing.