Back in the early days of the Clean Water Act, pond systems were the ticket for industrial production facilities located in remote areas where municipal treatment was not available. Today pond treatment systems are subject to more stringent discharge limits, often making them more of a burden than an asset. Operators, engineers, and equipment manufactures continue to come up with new and innovative ways to keep these treatment systems operational and within compliance. However, these data systems have significant challenges compared to modern mechanical treatment options.
To provide adequate conditions for wastewater treatment, aerobic treatment ponds need to be relatively shallow ponds, with a large surface area, to allow for adequate light penetration and water quality. As a result, these ponds take up massive amounts of space, especially when you consider the poor removal rates achieved through this method of treatment.
Compare the images of the aerobic treatment pond and the aeration tank below. What’s the biggest difference between them?
For the living organisms in the aerobic pond or tank to digest the nutrients(??) from the incoming wastestream, constant mixing is required.
It is common for facilities to utilize floating mechanical aerators to provide mixing, however accessing these aerators can be challenging. Many treatment plants keep a small boat on hand to access the mixers, providing a means to perform maintenance. For the facilities that do not have a boat or other means to access the aerators, maintenance can be far more costly. In these situations, it is necessary to have a crane called to the site to remove the equipment from the pond to perform maintenance or repairs, and then return the aerator(s) to the pond.
In addition to difficulties in accessing the aerators, power usage should also be a consideration. To operate the floating aerators, numerous motors are required for mixing – using a considerable amount of power, often in excess of 100 horsepower. During winter months in northern areas, these aerators can freeze in place and stop working or even flip over if they have an imbalanced snow load.
Alternatively, the blowers supplying the air to the pond can be used for mix the pond, assuming the correct aeration equipment is chosen. This method in itself has maintenance issues because the aeration diffusers or membranes need to be removed once every couple of years for inspection and repair. When using air to mix a pond, the engineers take into careful consideration the design mixed liquor concentration and often use a very narrow concentration in which the pond is designed to run. If the actual concentration of the mixed liquor in the pond exceeds the design specifications, it can be expected that the pond will no longer be completely mixed, and solids will settle at the bottom. This will cause a layer of anoxic or anaerobic sludge on the bottom of the pond that will cause other operational issues.
Aeration: Pond vs. Complete-Mix Aeration Basin
While a pond may require multiple aerators to provide adequate mixing, a complete-mix aeration basin will only utilize a single mix pump that is located outside of the tank and indoors, where maintenance can easily be performed. The in-tank part of the mix system is constructed of fiberglass material that will last in excess of 20 years before any type maintenance is required. Additionally, the amount of power required to provide adequate aeration in a tank is greatly reduced since only a single pump is utilized and the taller water column provide significant increases in efficiency.
Understanding the Challenges with Pond Systems
Despite the maintenance restrictions and power consumptions of pond systems, the biggest challenge is the lack of control. From a biological treatment stand-point, everything in wastewater works very slow. However pond systems react even more slowly because of the large volume of liquid. What can be accomplished in a several hundred thousand gallon tank may require millions of gallons to effectively treat in a pond.
These large volumes of water are more susceptible to ambient air temperatures than a tank because the hydraulic retention time is much longer. With a significantly smaller volume required in the tank, the smaller tank allows for quicker turnover and higher temperatures. (As warm production waters enter the tank, the small size and lower retention time allow the tank to stay warmer). In the cold northern winters, it is not uncommon for the temperatures in an aeration pond to get to near freezing temperatures, causing biological treatment to nearly stop because of the low temperatures. This can cause numerous possible discharge exceedances, including TSS, BOD, ammonia, and phosphorus.
To effectively utilize a treatment pond in a state like Wisconsin where phosphorus limits are getting increasingly stringent, the pond would have required being designed specifically for a certain level of phosphorus removal while accounting for winter weather. In situations where the design of an old pond may be inadequate, an anoxic zone can be added for bio-p removal, and air stripping can be added for ammonia removal. However, there comes a time when you have to ask if it makes financial sense to keep putting money into an aging treatment pond.
In the northern climates rain and snow can also be an issue. If you have a pond that is 350’ by 350’, every inch of rain equates to roughly 75,000 gallons of clean rainwater that needs to be processed through the treatment system. In Green Bay, WI the average rainfall per years is 29.5” per year. Using the fore mentioned pond size as an example, that equates to treating roughly 2.25 million gallons of clean rainwater per year. Or if you like to be a little more dramatic, the equivalent volume of 3.4 standard Olympic swimming pools.
Warm, Arid Climates
In warmer and drier climates such as west Texas or New Mexico, treatment ponds receive the benefit of a considerable amount of evaporation. In arid climates such as these, you can see as much as 30” of evaporation per year (or more) which reduces the overall amount of water that needs to be treated and discharged. The amount of evaporation can even be increased by adding splashing aerators on the surface of the ponds. The evaporation greatly reduces the amount of treated wastewater that then needs to be discharged.
One of the largest down sides to ponds in these arid southwest is the frequent debris that wind up in the pond. Tumbleweeds blowing across the plains aren’t going to stop just because they come up on a pesky fence, they just pile up until they can roll right over the top and into the ponds. High winds also blow a considerable amount of dust and dirt into the ponds which can build up over the years. Ever seen a Haboob? They are a giant dust storm that more or less looks like the end of the world is coming. While these aren’t terribly common in the southwest US, they do happen and leave an incredible amount of dust and dirt in their wake. To prevent this dust and dirt from building up in the treatment system you need to keep it mixed enough so these solids can be wasted out or they will eventually build up in the bottom of the pond and reduce the overall treatment volume.
Sludge and Sedimentation Build Up
That brings up the next issue; sludge and sedimentation cleanout. If the system has a settling pond as part of the process, it is specifically designed to settle solids out. These solids need to be cleaned out as they build up over time. In order to accomplish this cleanout without emptying the entire contents of the pond specialized equipment must be purchased or rented to crawl along the bottom and essentially vacuum out the solids. Alternatively, you can empty most of the pond and pump the remaining contents out, but either way it requires steps that are much more complicated than with a mechanical treatment plant.
While treatment ponds for industrial manufactures have their pros and cons, at the end of the day it comes down to what makes the most financial sense to meet regulatory requirements and good environment stewardship.
Whether your staff needs training to optimize their system, you need assistance with permitting or compliance requirements, your treatment system needs modifications to meet regulations, or you want to look at the option of building a mechanical treatment plant, The Probst Group can do all of the above.