While you might know that we need to treat the wastewater of our industries and cities, do you know what makes up water and wastewater? To understand why wastewater is considered a waste product, we first need to identify its constituents and why we need to treat them. Wastewater treatment is a close equilibrium between chemical and biological treatments that works in conjunction to remove these constituents. Here is a short but important list of parameters and terms that will allow you to better understand what your Aquasan representative is trying to achieve when designing a chemical treatment.
The pH, a seemingly simple parameter to control, is also a critical quality component of the system. The pH is a measure of the hydrogen ion concentration in water. The concentration range for most biological reactions is quite narrow and occurs in the pH range of pH–6-9. Wastewater outside this range is extremely difficult to treat by biological means, and if the concentration is not altered before its release into the aquifer, it will alter the natural water concentration, thereby affecting the natural environment. In addition, pH is also a critical variable in the design of a chemical program because some coagulants and polymers require a specific pH. Therefore, pH is one of the primary concerns with wastewater, as it directly dictates the performance of the biological system or the health of the aquifer into which the wastewater is released.
Alkalinity results from the presence of hydroxide [OH-], carbonates [CO3 2-] and bicarbonates [HCO3-] groups. Although alkalinity is important for resisting pH changes due to the addition of acidic compounds, it is a critical component in which chemical and biological treatment systems are used. Alkalinity is consumed for the biological removal of nutrients such as nitrogen, making it a critical parameter when planning biological nutrient removal.
Nitrogen is essential for the growth of microorganisms, plants, and animals, and acts as an essential building block for protein synthesis. Nitrogenous compounds are nutrients in many complex biological systems, including wastewater systems, meaning that both the lack and excess nitrogen are detrimental to the “treatability” of the water. Nitrogen, in its many forms, is often analyzed and quantified to ensure that biological nitrogen removal can be achieved to the fullest extent, since excess nitrogen in the effluent could ultimately lead to eutrophication of the downstream ecosystem.
Phosphorus is also an essential nutrient for the growth of organisms and, more specifically, for the growth of algae. Algal blooms are a primary target for many municipalities and regulating bodies because they lead to the depletion of dissolved oxygen in the water body, causing eutrophication. Phosphorus is also analyzed and quantified when designing a chemical program since phosphorus, under certain forms, can be treated biologically and, contrary to nitrogen, can also be treated chemically with the addition of coagulants.
Aggregate Organic Constituents (BOD/COD/TOC)
Organic compounds in wastewater usually consist of proteins, carbohydrates, and FOGs (fats, oils, and greases) but also have a number of different synthetic organic molecules that range from simple to extremely complex. Organic compounds of various complexities also have different tendencies to decompose over time. It is often simpler and more accurate to look at all specific constituents as the sum of all organic matter or as aggregate organic constituents. These organic compounds need to undergo decomposition in the natural ecosystem if they are not addressed before their release into the environment. To decompose these compounds, organisms will consume the organic matter in 3 different pathways: oxidation, synthesis, and endogenous respiration. The common factor for these three pathways is the net consumption of dissolved oxygen, leading to the depletion of oxygen in the aquifer and ultimately the reduction of life in the aquifers.
There are 3 laboratory methods commonly used to measure gross organic matter biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total organic carbon (TOC).
Even if the BOD/BOD5 test presents many limitations, it is still widely used to size wastewater treatment plants as well as to determine the compliance of these plants with discharge permits, making it critical to understand the term and its value. Determination of the 5-day BOD (BOD5) involves the measurement of dissolved oxygen consumed by microorganisms in the biochemical oxidation of organic matter. This concept can also be simplified to: if sufficient dissolved oxygen is available, the aerobic biological decomposition of organic waste will continue until all of the waste is consumed. For the effluent, a low BOD concentration indicates less contaminated water, resulting in less depletion of oxygen concentration as organisms decompose the rest of the organic compounds in the natural aquifer.
COD, or Chemical Oxygen Demand (COD), is a test that measures how much oxygen equivalent of the organic material in the wastewater can be oxidized chemically using a solution of acid and dichromate. While this test is representative of a larger part of the total organic matter present (chemically oxidizable), it is often used operationally to determine the amount of organic matter present in wastewater because of the short span of time necessary to achieve this test. As a comparison, the COD test takes around 2.5 hours to produce, while the BOD5 test is a 5-day test. Usually, the COD test is used as a ratio to determine the BOD5 rapidly without having to wait 5 days for the results. Usual BOD:COD ranges are 0.3 to 0.8 for untreated effluent and around 0.5 for wastewater that has undergone primary treatment.
TOC, or total organic carbon, is a test that basically transforms the total organic carbon of a solution into CO2 by multiple means, such as heat, UV, oxygen, chemical oxidants, or a combination of each of these. The CO2 was then analyzed and quantified to determine the TOC load. TOC is also operationally popular because it can be achieved within 10–15 min. However, a valid relationship must be established between TOC and BOD to be able to use the results of the TOC test. The TOC test should not be used on rapidly changing wastewater. Usual BOD:TOC ranges are 1.2 to 2.0 for untreated effluent and around 0.8 to 1.2 for wastewater that has undergone primary treatment.
FOGs (Fats, Oils and Greases)
As the name implies, FOG represents the fat, oil, and grease portions of wastewater constituents. The main contributor to FOG is domestic usage of fats and oils. Another major contributor is the industrial transformation of foods, where large amounts of fat are concentrated in a single watershed. Lastly, petroleum and tar derivatives are also present in water streams from garages, shops, or streets. If the FOG is not removed from the water before discharge to the natural waters, it will create a film on top of the aquifer, disrupting the exchanges necessary for the maintenance of biological life. It is crucial to eliminate all the FOG before discharge because a minuscule amount will create a thin film that is detrimental to biological life. Rates as low as 0.5 liters of FOG per hectare of surface water create a thin film that is powerful enough to disturb aquatic life.
Trace amounts of many metals are present in wastewater, and many of these metals are necessary for the growth of microorganisms. However, we must ensure that these metals do not reach toxic concentrations in the effluent streams, as some metals tend to accumulate or bioaccumulate in the ecosystem. The metal should be analyzed in two different streams: water and sludge. When we dewater sludge to compost and spread biosolids onto land, we must be extremely vigilant for the tendency of some metals to accumulate and concentrate at the point of toxicity. Most environmental legislative bodies have strict requirements for land application of biosolids to prevent the accumulation of metals on arable land.