Requirements for biological treatment equipment, Part 1: Trickling filters
1977 Edition

Recommended filter media include locally sourced crushed stone or gravel, considered most appropriate for use. Alternative materials such as slag, cinders, and hard coal may also be employed. The media must be inert, chemically stable in wastewater, and resistant to physical breakdown under continuous exposure and temperature fluctuations. In colder climates, natural stone media should undergo the sodium sulphate soundness test to evaluate durability. Media failing if more than 10% by mass of samples degrade or if over 10% passes through a 12.5 mm sieve after testing should be rejected. Materials with significant silica content require additional soundness verification to ensure long-term performance.

The underdrainage system consists of filter blocks acting as lateral drains, supported on piers made of cement concrete, with a main drainage channel collecting the effluent for discharge. Ventilators are installed around the entire filter perimeter at 2-meter intervals, each with a diameter exceeding 100 mm and elevated 75 mm above the filter media surface. These ventilators are fitted with cowls to prevent debris and insect ingress while allowing air circulation. The design facilitates uniform effluent collection and ensures sufficient aeration beneath the filter bed to prevent anaerobic conditions, thereby maintaining treatment efficiency.

Hydraulic reaction-driven rotary distributors rotate due to the force of wastewater discharged from orifices on their arms, with speed varying based on flow conditions, typically around 2 revolutions per minute for two-arm units and 1.4 rpm for four-arm units. They feature a central assembly with a stationary turntable base and ball bearings sealed to prevent leakage. In contrast, motor-driven rotary distributors utilize a fixed-speed electric motor coupled with a reduction gear to maintain constant rotation speed, irrespective of flow. This design requires additional structural provisions such as walkways for motor maintenance during operation. Both types employ oil-bath ball bearings with sealing arrangements but differ primarily in their driving mechanism and maintenance access.

Ventilators must be placed continuously along the filter bed perimeter with a center-to-center spacing of 200 centimeters (2 meters). Each ventilator should have a diameter greater than 100 millimeters and be positioned 75 millimeters above the top surface of the filter media. Additionally, ventilators need to be equipped with a protective cowl to prevent entry of debris and insects, ensuring unobstructed airflow. This configuration promotes uniform aeration throughout the filter bed, which is essential for maintaining aerobic conditions and optimizing treatment performance.

Steel components must first be thoroughly cleaned to remove all rust, grease, and moisture, ensuring a dry surface. Subsequently, an anticorrosive primer coat should be applied, followed by at least three coats of bitumastic or equivalent anticorrosive paint after installation. Alternatively, epoxy or similar protective coatings may be used to enhance corrosion resistance. These procedures comply with IS 1477 Parts I and II and provide durable protection for steel parts such as tic rods, distribution arms, and structural steel elements exposed to the moist environment within trickling filters.