6 wastewater parameters and dealing with discharge limits

Here we will go through the 6 main wastewater parameters and how to deal with environmental discharge limits.

One of the major obstacles that our clients face is ‘Understanding my environmental discharge limits for wastewater’ This is something they face whether it is getting approval on their future project or it could be a change to operational parameters that the government changes based upon new regulation. Either way Alumichem is here to help. 

In today’s environmental framework we are typically looking at 6 main parameters that the discharge limits are focused on: Total Suspended Solids (TSS), Chemical Oxygen Demand (COD), Biological Oxygen Demand5 (BOD5), Total Nitrogen (Total-N), Total Phosphorous (Total-P) and Heavy Metals. To be able to understand how to reduce and control these parameters we first need to do our best to understand what each of these parameters is.

TSS in wastewater treatment

Total Suspended Solids (TSS) is defined as the dry weight of all suspended particles that are not in a dissolved state that can be trapped in a filter with a mesh size of 0.45 micron. In the world of aquaculture and fish processing this can be undisgested food, feces, scales, oils & fats, blood and microbiological contaminants.  Other names that may be used to describe TSS are Total Suspended Matter, Suspended Particulate Matter or historically it was called Non-Filterable Residue.

Note: In seawater samples it is necessary to ‘wash’ the filter and sample with deionized water after filtering and before drying to remove the influence of salts left on the filter during drying.
The removal of TSS is typically achieved through physical treatment processes such as sedimentation, filtration, and flotation. Sedimentation involves allowing the suspended solids to settle out of the water by gravity, while filtration involves passing the water through a filter media to remove the solids. Flotation is a process in which air is introduced into the water to create bubbles, which attach to the suspended solids and carry them to the surface where they can be removed.

COD in wastewater treatment

The purpose of understanding Chemical Oxygen Demand (COD) is to try and provide an indicative measure of the amount of oxygen that can be consumed by reactions in the water stream.  Essentially COD is indicative of the amount or organic matter in the water that will ultimately be reacting with oxygen (O2). The premise being that nearly all organic compounds can be fully oxidized to carbon dioxide (CO2) by O2 and therefore it gives an indication of the amount of excess O2 that will be needed in the receiving water body to breakdown the organic compounds.  The amount of O2 required to oxidize an organic compound to CO2, ammonia (NH4) and water (H2O) is expressed by:

There are several methods for reducing COD levels in wastewater treatment, including biological treatment, chemical treatment, and physical treatment. Biological treatment involves the use of microorganisms to break down organic matter in the wastewater, while chemical treatment involves the use of chemicals to oxidize or reduce the organic matter. Physical treatment methods involve the use of physical processes, such as sedimentation or filtration, to remove the organic matter from the wastewater. Overall, the choice of treatment method will depend on the specific characteristics of the wastewater and the treatment goals.

BOD in wastewater treatment

Biological Oxygen Demand5 (BOD5) is the amount of dissolved oxygen required by aerobic biological organisms to break down the organic material present in the water over a period of 5 days at a temperature of 20°C. BOD5 in an effluent is used to indicate the short-term impact on the oxygen levels of the receiving water body. BOD5 is directly related to COD in that they both measure the amount of organic contamination in the water however, the analysis of COD is more generic in that it measures all organics that can be oxidized rather that just the organics that can be biologically oxidized.

High BOD levels in wastewater can lead to a depletion of oxygen in the receiving water bodies, which can result in fish kills and other ecological problems. Therefore, it is important to remove as much organic matter as possible from wastewater before discharging it into the environment.

To remove organic matter and reduce BOD levels in wastewater, a variety of treatment processes are used, including physical, chemical, and biological methods. These may include screening, sedimentation, biological treatment, and disinfection.

Total Nitrogen (Total-N)

Before we can look at Total-N we first have to understand how Nitrogen (N) is present in water because Total-N is the sum of the different forms of N in the water:

Ammonia (NH3)
Nitrite (NO2)
Nitrate (NO3+)
Organic Nitrogen (Organic-N)

Ammonia-N = NH3 + NH4+ (pH and temperature related)
Total Kjeldahl Nitrogen (TKN) = Ammonia-N + Organic-N
Total Inorganic Nitrogen (TIN) = Ammonia-N + NO2 + NO3+

Total-N = Ammonia-N + NO2 + NO3+ + Organic-N

The unfortunate part of nitrogen in the effluent is that the forms of dissolved Nitrogen are difficult to remove from the system and secondly the dissolved forms tend to be toxic to either aquatic life or plant life.  Therefore, the environmental concerns regarding the release of dissolved Nitrogen in the effluent is becoming greater.

Total-N is often removed through a process called biological nitrogen removal. This process involves two stages: nitrification and denitrification. During nitrification, bacteria convert ammonia (NH3) to nitrite (NO2-) and then to nitrate (NO3-). During denitrification, other bacteria convert nitrate to nitrogen gas (N2), which is then released into the atmosphere.

There are several factors that can affect the efficiency of biological nitrogen removal, including temperature, pH, dissolved oxygen levels, and the concentration of organic matter in the wastewater. Monitoring the Total-N levels in the influent and effluent of the treatment process is important to ensure that the treatment system is operating effectively and meeting regulatory requirements

Total Phosphorous (Total-P)

Total Phosphorous (Total-P) is a measure of all of the phosphorous found in the water whether it is in particulate form or soluble form.  When Phosphorous is present in the waste stream it can be found in the ionic form, PO43-, or in the inorganic form, orthophosphates and polyphosphates, or in an organic form, organically bound phosphates.  Regardless of the form the Phosphorous is in it will translate into increased algae growth and aquatic plant growth to the point that the growth rates can choke the oxygen from the recipient water source and cause Eutrophication and ultimately cause the ecosystem to completely change.  This is the main reason that discharge limits are placed upon our clients.

Dealing with Heavy Metals in discharge water

Heavy metals in the discharge water in the Aquaculture and fish processing industry can become a concern depending on what is put into the system. It is important to remember that anything that is added has to come out somewhere, either in the fish, in the water or in in the sludge.  Heavy metals become a bigger issue in intensive recirculating systems as the heavy metals can concentrate due to the low level of new water being introduced into the system.  Regardless of this situation we will discuss what are heavy metals for understanding.

The most common heavy metals that environmental authorities are concerned about are lead (Pb), zinc (Zn), mercury (Hg), nickel (Ni), cadmium (Cd), copper (Cu), chromium (Cr), and arsenic (As). Other than the short- and long-term toxicity concerns with regards to heavy metals, some negative impacts of heavy metals to aquatic ecosystems include death of aquatic life, algal blooms, habitat destruction from sedimentation, and debris.

Besides in the water discharge, heavy metals can be collected with the precipitated solids and end up in the sludge. This is important to understand as some countries have legislation placing sludge in different categories depending on heavy metals content. This categorization governs what reuse options are allowed.

Now that you have a grasp on what the contaminants are that Environmental legislation is looking at we can look at the best ways to deal with the parameters.  For this please take a look at our previous blog Aquaculture Effluent Treatment Guide –>


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