When comparing biological and chemical wastewater treatment strategies, how do you know which technologies will best suit your facility’s treatment needs?
Fundamentally, biological treatment uses living bacteria to do the bulk of the purification while chemical treatment uses chemical reactants to accomplish the work, and at first glance, the comparisons seem straightforward—but what contaminants do each of these processes remove? How do they compare in cost, footprint, energy use, and operation? Should your facility be using just one of these processes? Or should it be using both?
In this article, “Biological vs. Chemical Wastewater Treatment: Which Is Better for Your Industrial Facility?”, we examine the key differences between the two wastewater treatment methods, including how they work and typical applications, so your facility can be sure it’s properly using (or planning to use) the technologies ideally suited for its specific processes and treatment needs.
Which pollutants do biological and chemical wastewater treatment systems remove?
When it comes to understating which technologies might make up your wastewater treatment system, one important factor in making that decision will come down to the type of contaminants that need to be removed. These are some of the ways biological and chemical treatment can help.
Biological treatment systems
Biological wastewater treatment systems are best at removing things that are easily biodegradable. For example, things used in your garden or kitchen that are made up of carbohydrates, polysaccharides, starches, sugars, proteins and edible fats (like lipids or plant-based oils). To this list, we can also add hydrocarbons (like ethane, hexane, pentane, octane, and nonane), as well as smaller aromatic compounds (like benzene, toluene, xylene, and simple phenols) which are also simple compounds that are easy for bacteria to break down and digest.
Chemical treatment systems
Pollutants removed with chemistry are typically toxic soluble metals (like aluminum, cobalt, copper, iron, mercury, nickel, lead, and zinc). Bacteria aren’t effective at removing these contaminants, but treating these metals chemically by adding either calcium hydroxide or sodium hydroxide helps form insoluble metal hydroxides that can precipitate out of the water solution.
Industries like mining and steel-making or oil and gas—wherever a facility is using raw materials from the earth that, as a byproduct, have a wastewater bearing these kinds of compounds—will generally use chemical rather than biological wastewater treatment technologies.
Biological and chemical treatment systems
If your facility’s contaminants are more complex, like certain polymers or polynuclear links, etc., these tend to be harder for the bacteria to degrade. In these cases, chemistry can be used in the preliminary wastewater treatment phases to break down nondegradable compounds by oxidizing them partially and converting them from their polymer (or more complex form) to their monomer or dimer (or simpler form) so the bacteria can take over and treat the rest of the broken-down molecules from there. In these cases, you might use advanced oxidation chemistry, like ozone or hydrogen peroxide, to precede the biological component.
A combination of chemical and biological wastewater treatment might be required where industrial streams are highly concentrated with hydrocarbons, such as in certain refineries. These problematic streams are segmented out for chemical pretreatment—where a bulk of the toxicity is removed—then join the streams going to the biological plant, but this doesn’t happen often.
There are a few cases where you must choose between either biological or chemical treatment, for example with removing selenium. Historically, it has been removed chemically by adding reducing agents and adsorbents, but it can also be removed biologically with anoxic nitrate-reducing bacteria.
What are the differences in system processes?
The technologies of choice are largely dependent upon the pollutants that need to be removed. As mentioned above, each of these methods only removes certain pollutants efficiently, so you’re often required to use one technology or the other.
Although treatment methods can vary, in biological wastewater treatment, the standard process includes:
- pretreatment to remove particulates or insoluble solids;
- biological treatment, which is typically aerated but could also be anoxic (treatment methods could be anaerobic where you’re fermenting down to methane, CO2, and biogas), but 95% of biological wastewater treatment is aerobic. Biological wastewater treatment uses technologies like moving bed bioreactors, fixed bed bioreactors, membrane bioreactors, biological trickling filters, rotating biological contactors, sequencing batch contactor, etc. In the biological world, you have the whole microbial kingdom to draw upon to accomplish your treatment objectives;
- posttreatment to clarify out bacterial solids to further purify the water.
On the other end, chemical wastewater treatment standard processes typically include:
- precipitation of the metals
- coagulation and flocculation; after all the larger objects settle out, various chemicals are added to a reaction tank to remove the bulk suspended solids and other various contaminants. This process takes out all the finer particles in the water by combining them into heavier particles that settle out. The most widely used coagulates are aluminum-based such as alum and polyaluminum chloride. Sometimes a slight pH adjustment will help coagulate the particles, as well.
- ion exchange, which is common for drinking water applications or any other applications that require high-quality water (like recycling the water for use in other processes or making steam)
- disinfection, where chemicals like chlorine remove any kind of bacteria that may be in the wastewater that needs to be disinfected before it’s discharged
In chemistry, though, you might have a few stages depending upon the chemical reactions and what’s in the water, like adjust the pH or temperature before adding chemicals. The key objective, here, is to mix the chemical reactants with the wastewater pollutants thoroughly to optimize chemical reactions and kinetics in order to complete the treatment as quickly and efficiently as possible.
What are the differences in cost, footprint, and energy use?
With chemical treatment, it’s important to have the right amount of reactant in order to get the reactions necessary, and typically you’re paying for every kilogram of chemical reactant you need. In contrast, bacteria multiply on their own, so once you get them started and they begin proliferating in the ideal growth environment, as long as they’re happy, they’re working a bit harder for you. With this in mind, you can easily see why it is usually more affordable to use bacteria on a normal day-to-day cost basis.
However, installing the biological system can be higher up front. You will still need tanks, mixers, and pumps for your chemical reactants, but that is usually a lower capital cost than a biological system that has aeration, pH control, temperature control, maybe some packing media, etc. Therefore, while biological systems will be a higher capital cost to install and get started, they generally have a much lower operating cost than chemical wastewater treatment.
Typically, in biological systems, the electrical costs are going to be the highest contributing factor in your operating cost. Whereas in chemical treatment, your chemicals are always the highest contributing component to the operating cost. Also keep in mind it often requires a higher amount of manpower to operate a chemical plant versus biological.
The design of any treatment system will depend upon the characteristics, flow rates, and target effluent limits you’re trying to achieve of the water. Also, the bulk of the pollutant removal might require a larger footprint for a biological treatment system than the corresponding chemical treatment system.
So, which type of treatment system is best for our facility?
As always, we suggest you consult your water treatment specialist to ensure the technologies your facility pursues are the right fit for your process and needs.
But in general, your choice will depend on the:
- wastewater profile
- flow rate
- contaminant levels in relation to how low the levels need to be after treatment
- land and utilities available
For example, a gold mine in remote areas that doesn’t have access to the proper utilities might not be able to use biological treatment, and chemical may be preferred in that case. Biological systems, in general, may require pumps and aeration or steam, which would all use considerably more electricity than with chemistry, where you may not need as much steam to keep things in the right balance.
Therefore, it’s important to weigh all these considerations when designing your wastewater treatment system and address these issues with your water treatment specialists so they can better guide you toward making the right treatment choices.
Can SAMCO help?
SAMCO has over 40 years’ experience custom-designing and manufacturing biological wastewater treatment systems, so please feel free to reach out to us with your questions.
Our biological treatment solutions—including FBBR, MBR, MBBR, and activated sludge solutions, among others—can help your facility:
- decrease its footprint, chemical use, and energy consumption
- produce high-quality effluent
- recycle organically contaminated water
- manage high levels of BOD and difficult-to-treat wastewaters
For more information or to get in touch, contact us here. You can also visit our website to set up a call with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your biological wastewater treatment system needs.