Question

Estimate the total organic carbon (TOC) concentration of waste water whose chemical oxygen demand (COD) is \(500 \mathrm{mgL}^{-1}\left(\mathrm{O}_{2}\right)\). What fraction of the total (dissolved and particulate) solids content of \(720 \mathrm{mg} \mathrm{L}^{-1}\) is then made up of organic material? Assume the organic fraction can be represented as \(\left[\mathrm{CH}_{2} \mathrm{O}\right]\). Of what might the remaining solids consist?

Short answer

TOC is 187.5 mg/L, with 26.04% of solids being organic; the rest are inorganic materials.

Step-by-step solution

Understand the Relationship Between COD and TOC

The COD measures the amount of oxygen required to oxidize organic matter in water, while TOC quantifies the carbon. The relationship assumes that 1 gram of \[\text{COD} (O_2)\] consumes 0.375 grams of organic carbon \[(CH_2O)\]. This is the stoichiometric relationship for organic molecules like carbohydrates that have the formula \[CH_2O\].

Calculate the TOC from COD

Given COD = 500 mg/L \[O_2\], we use the relationship \[1 \, \text{mg} \ O_2 = 0.375 \, \text{mg} \, C\] to calculate the TOC. \[\text{TOC} = 500 \, \text{mg/L} \, \times \, 0.375 = 187.5 \, \text{mg/L}\] of organic carbon.

Calculate Organic Fraction of Solids

The total solids concentration is given as 720 mg/L. Using the TOC value calculated: \[\frac{\text{TOC}}{\text{Total Solids}} = \frac{187.5}{720} \times 100\% = 26.04\%\].This gives us the percentage of the total solids that is organic material.

Discuss Composition of Remaining Solids

The remaining 73.96% of the solids are made up of inorganic material such as suspended particles, silt, sand, and possibly salts or minerals.

Key concepts

Total Organic Carbon (TOC)

Total Organic Carbon, or TOC, is a key metric in environmental chemistry used to measure the amount of carbon in organic compounds present in water or wastewater. It is an essential parameter as it provides insight into the level of organic pollutants, which can impact both aquatic life and human health. TOC is expressed in terms of milligrams of carbon per liter (mg/L). It represents compounds that can range from small compounds like sugars and alcohols to larger molecules such as natural polymers or synthetic detergents.

In wastewater analysis, TOC is crucial since it not only helps identify pollution sources but also assists in evaluating treatment processes. By understanding TOC levels, environmental engineers can design and optimize treatment systems to reduce pollution effectively.

• High TOC levels indicate higher amounts of organic pollution.
• TOC monitoring aids in compliance with environmental regulations.
• It helps in assessing the effectiveness of water treatment processes.

Typically, TOC is measured using specialized equipment such as TOC analyzers, which oxidize organic carbon to CO2 and measure it. This parameter, combined with other analyses like Chemical Oxygen Demand (COD), offers a comprehensive view of water quality.

Chemical Oxygen Demand (COD)

Chemical Oxygen Demand, or COD, is a vital parameter in wastewater analysis. It measures the amount of oxygen required to chemically oxidize organic and inorganic compounds in water. COD is typically expressed in terms of milligrams of oxygen per liter (mg/L).

This parameter is especially useful because it provides an estimate of the total amount of organic material that can be oxidized, thus offering a measure of potential pollution. The higher the COD, the more oxygen is needed, indicating a higher level of pollution.

• COD testing helps in monitoring and controlling water pollution.
• It provides data to ensure wastewater meets discharge requirements.
• It's often used together with biochemical oxygen demand (BOD) analysis to get a full picture of water quality.

In practice, COD is determined by adding a strong oxidizing agent (like potassium dichromate) to the water sample, followed by heat application. The change in the amount of oxidant reflects the COD, which in turn indicates the level of organic compounds present.

Wastewater Analysis

Wastewater analysis involves the evaluation of water that has been used and contaminated in homes, industries, and businesses. This analysis is crucial for understanding the quality of wastewater and determining appropriate treatment methods. It involves various tests, among which TOC and COD are essential.

This process helps environmental scientists and engineers assess:

• The type and concentration of pollutants.
• The efficiency of existing water treatment systems.
• The potential environmental impact of discharging untreated wastewater.

By understanding these factors, professionals can design better wastewater treatment processes to remove contaminants effectively and safely. This is critical to protect water quality and ensure compliance with environmental regulations. Furthermore, analyzing wastewater provides data that can be used to optimize processes for recycling or reusing treated water, promoting sustainable water management practices.

Stoichiometric Calculations

Stoichiometric calculations in environmental chemistry involve using balanced chemical equations to calculate the relationships between reactants and products. In the context of TOC and COD, stoichiometry helps determine how the amounts of these components relate to each other under given conditions.

For example, in the provided exercise, the relationship between COD and TOC is derived from a stoichiometric ratio. It is based on the assumption that organic matter, represented as \(CH_2O\), reacts with oxygen. Specifically:

• 1 gram of COD (\(O_2\)) is estimated to consume 0.375 grams of organic carbon (\(CH_2O\)).

The stoichiometric relationship allows us to convert COD values to equivalent TOC values, aiding in the assessment of water quality. Accurate stoichiometric calculations are crucial in designing and implementing effective wastewater treatment strategies. By understanding the quantities involved, engineers can predict outcomes and optimize treatment processes for maximal effectiveness in reducing organic pollutants.