All chemical reactions are time dependent, some occurring quickly and others slowly. It also is well known in water treatment that one must consider and evaluate for design the residence (or "retention") time in the "reactor" that is needed to achieve the necessary response. This is true also for any treatment approach to AMD and metals, whether it is a classical alkaline amendment, or a biogeochemical approach such as in-situ permeable barrier using either a carbon-based or a zero-valiant iron system.

Settling ponds will work, sometimes very well, for reducing TSS. However, ponds alone will not address either AMD or the metals in solution that are pH dependent. You need to change the chemistry of the system. Given that you are amending the chemistry in some fashion, strong the reacted fluids in settling ponds to allow precipitates to settle is an approach to dealing with retention time, and it is one that may be effective if you have very fine-grained reaction products that settle slowly.

Generally we use jar tests to determine reagent dosage and mixing time on the lab bench, filter the samples and submit the solids and solution for chemical analysis. Different flocculants may also be screened during the bench scale testing. Bulk sample precipitation comes next with the solid: liquid separation studies for preliminary process design and continuous bench scale testing. Success leads to pilot testing to generate enough sludge for waste disposal alternative evaluations.

When using ponds as the sole source of precipitating metal, I highly recommend a duplicate system, by which as one set of ponds fill up (thus the detention time is reduced), the other set can be used. This way the filled ponds can be decanted, allowing the sludge to dry and greatly reduce the volume and ease of removing the material.