Author: Vanessa Friesen, PhD., RPBio., Lead, Biological Water Treatment, Ensero Solutions
This article is part 1 of a 3-part series on biological water treatment for mine impacted water. It is a reference document for planners, engineers, and operators so they can better understand terminology used by biological treatment equipment vendors and engineering/ environmental design consultants. Upcoming articles will provide more information on engineering design considerations and provide project examples.
Biological (Bio) approaches to water treatment are an innovative and often cost-effective part of the technology toolbox for mine and power operations, closure planning, and site remediation. Bio treatment approaches allow owners to implement treatment that can be specifically tailored to the site needs.
Why is it Called Biological Treatment?
It’s called Bio treatment because the technology relies on biological activity to remove constituents of interest (COIs) from water. Microbes commonly provide the mode of action in bio treatment systems. COIs are removed from the environment either degraded by the microbe (typically used for food and metabolized) or the microbe facilitates a chemical reaction that results in the COIs being precipitated out of solution.
When thinking about Bio treatment systems, active tank-based bioreactors commonly come to mind. However, several other Bio treatment systems are available and effective in treating impacted water. Options are categorized several different ways, based on the site location/condition and the level of operational expenditure (OpEx) required for the system. These categories guide site-specific technology selection and are defined below to provide clarity.
Passive, Semi-Passive, and Active Treatment
The terms passive, semi-passive, and active treatment are used to describe the level of operation and maintenance/oversight needed for the system. Although these treatment systems can be based on physical or chemical reactions rather than biological reactions, the examples discussed here are biological.
Passive Bio water treatment systems support the most “green” treatment approaches. They are designed to operate with no or little electricity (if electricity is needed, solar or wind are used); operate with no or minimal reagent additions after start-up and require minimal routine maintenance. In some cases, periodic replacement of substrates can be needed depending on the system design and longevity. Examples of passive Bio treatment systems are simple constructed wetland treatment systems (CWTS), some organic-containing permeable reactive barriers (PRBs) or biochemical reactors (BCRs), and certain in-situ treatment designs discussed further below. Passive systems are typically the least expensive to operate over the long-term. Potential shortcomings include having sufficient area to operate (such as CWTS) or if high concentrations of COIs need to be removed.
Semi-passive water treatment combines the benefits of passive and active treatment systems. The idea behind semi-passive water treatment is to require less energy, operation, and maintenance than traditional active treatment systems but provide more operational control than passive treatment systems alone. These systems typically require multiple reagent additions, physical mixing, or mechanical rather than natural aeration. Examples of semi-passive Bio treatment systems are in-situ treatment within a pit lake, saturated rock fills (SRF), in situ groundwater injection (groundwater injection), and some biochemical reactor designs including gravel bed bioreactors (GBRs).
Active bio treatment systems are typically tank-based systems that require energy and daily operation and maintenance including reagent dosing. In the mining and power sectors they are commonly integrated at the end of a chemical and/or physical treatment process to remove constituents such as nitrate, nitrite, ammonia, selenium, and some metals. Examples of active Bio systems include biochemical reactor designs including some GBRs, moving bed biofilm reactor (MBBR), membrane bioreactor (MBR), fluidized bed reactor (FBR), and packed bed reactor (PBR).
In-situ versus Ex-situ Treatment
Simply put, the terms in situ and ex situ are used to describe where the water is treated. Water is either treated in place (in situ), like in a pit lake, flooded mine workings, saturated rock fill, or groundwater; or it is extracted and treated in a centralized location (ex situ). In-situ treatment commonly has lower capital expenditure (CapEx) and operational expenditure (OpEx) requirements than ex-situ treatment and requires less operation and maintenance effort.
Finding the right combination of these Bio treatment considerations – amount of OpEx, CapEx, and treatment location – allows mine owners to satisfy the site-specific project need(s), such as minimizing OpEx or CapEx, treatment schedule, physical constraints, or discharge limits.
Why does this matter?
The reliability and operational requirements of all Bio systems are highly dependent on technology selection and system design. When selecting a water treatment technology, there are several factors to consider, such as water quality effluent criteria, source water quality, reliability, existing conditions, process flexibility, longevity, site characteristics (physical space and climate), utility capabilities, and costs.
Assuming the selected technology is appropriate for the site COIs, site owners will benefit from understanding that many bio approaches can be implemented in a way that minimizes OpEx while still being effective. Selecting scientists, engineers, and vendors who understand the benefits and risks of Bio treatment systems increases the probability of meeting the site’s water quality objectives and the owner’s operational needs.