Designing a Neutralization or pH Adjustment Waste Treatment Mixing System:
Waste neutralization or pH adjustment tanks can be either a batch or continuous processing. We've indicated a common poly-tank such as that constructed of polyethylene or polypropylene in that these designs offer a wide chemical resistant range as well as are quite economical. Waste and water treatment facilities are typically concrete rectangular basins. In either case, the selected mixer is suitable for processing up to 12,000 gallons or less.
COMMON PROBLEMS: PROCESS CONTROL
Typical retention times for continuous pH adjustment or neutralization range from 1 to well over 100 minutes, where a target volumes are typically above 10 minutes. Generally speaking, a retention time less than 10 minutes is going to require additional energy per unit volume to properly process and ensure the chemical reaction. Although every mixer has some ability to suspend solids, if solid suspension is a requirement, parameters such as solid size, solid density, and percent by weight by mass will need to be identified. In many applications, sand maybe present where its eventual accumulation is negligible. Since the energy requirement to suspend a heavy solid like sand will significantly influence both the bottom line and energy requirement, in most cases, it is allowed to accumulate with infrequent manual removal.
Vertical tanks, with height to diameter ratios of 1.0 to 1.2 are preferred over horizontal tanks since they take up less space, are easily supported on a concrete slab and are simpler to mix. Tanks having mixers installed vertical-on-tank-centerline generally do not require anti-swirl baffles due to the process viscosity of the application. If you are using a lower viscosity solution, three (3) anti-swirl baffles installed 120 degrees apart, maybe required to prevent foaming and other considerations. Contact your mixer supplier for their recommendation.
As for the mixer design, the use of an upper impeller maybe recommended, especially if a solid feeding system such as a funnel eductor is used. In the event of dewatering, the upper impellers purpose will be to capture and entrain the dewatered top phase and pump it downward quickly into to lower regions of the storage tank to achieve a solution. Without the upper impeller, the lower viscosity water phase may stratify on the surface and take time to reincorporate into the batch exposing the solution to over mixing.