Question

A community intends to use their sewage waste (using urine separation) to derive energy from it. The material is very wet, however, and even with urine separation, only about \(29 \mathrm{wt} \%\) dry matter can be obtained. About 700 households produce \(125 \mathrm{~kg}\). day \({ }^{-1}\) of feces matter, and the combustion process that is to be applied requires 80 wt \(\%\) dry matter material. a. How much water must be removed per second in a continuous process? b. In case no heat losses occur, what would be the energy needed for this drying process? Neglect the heating up of the feces, but only consider the water heat up and evaporation at atmospheric conditions. c. Would the assumption in (b) lead to a large difference in the calculated heat supply?

Short answer

Answer: The process for calculating the energy needed for the drying process considering only water heat up and evaporation at atmospheric conditions involves the following steps: 1. Calculate the mass of dry matter produced per day from the moist feces matter. 2. Find the mass of moist feces needed to achieve the required dry matter percentage. 3. Calculate the mass of water that must be removed per day. 4. Calculate the amount of water to be removed per second for a continuous process. 5. Calculate the energy needed for water heat up and evaporation by adding the energy required for heating the water (Q_heat) and the energy required for water evaporation (Q_evaporate). 6. Discuss the assumption made and its potential difference on the heat calculation.

Step-by-step solution

Calculate the mass of dry matter produced per day

First, let's calculate the mass of dry matter produced per day from the moist feces matter. The mass can be found by multiplying the percentage of dry matter in feces by the mass of feces produced daily. Given: Total mass of feces (m_total) = 125 kg/day Dry matter percentage (p_dry) = 29 wt % Dry matter mass (m_dry) = m_total * p_dry / 100

Calculate the mass of moist feces needed to achieve the required dry matter percentage

The combustion process requires 80 wt% dry matter content. Therefore, we need to find the mass of moist feces (m_moist) that would provide us this percentage using the mass of dry matter found in Step 1. Desired dry matter percentage (p_desired) = 80 wt % m_moist = m_dry / (p_desired / 100)

Calculate the amount of water to be removed per day

Now that we have the mass of moist feces required, we can calculate the mass of water that must be removed per day (m_removed) using the mass of initial total feces and the mass of desired moist feces calculated in Steps 1 and 2. m_removed = m_total - m_moist

Calculate the amount of water removed per second

For a continuous process, we need to calculate the amount of water to be removed per second (m_removed_s) from the daily amount calculated in Step 3. There are 86400 seconds in a day. m_removed_s = m_removed / 86400

Calculate the energy needed for water heat up and evaporation

To calculate the energy needed (Q), we will only consider the water heat up and evaporation at atmospheric conditions. We will assume no heat losses. The energy needed for the drying process can be calculated by adding the energy required for heating the water (Q_heat) and the energy required for water evaporation (Q_evaporate). Q = Q_heat + Q_evaporate Let's use the mass of water removed per day for these calculations (m_removed, from Step 3) For heating water: Specific heat capacity of water (c_water) = 4.18 kJ/kg.K Temperature difference (dT) = 100 - 25 = 75 K (assuming water temperature rises from 25°C to 100°C) Q_heat = m_removed * c_water * dT For water evaporation: Latent heat of vaporization (L_water) = 2260 kJ/kg (at 100°C, 1 atm) Q_evaporate = m_removed * L_water

Discuss the assumption made in part b

The assumption made in part b regarding neglecting the heating up of feces might lead to a slight difference in the heat supply calculation. The specific heat capacity of the feces material is often much smaller than that of water, and as a result, this assumption is reasonable for a rough estimation. However, for a more accurate calculation, the energy required for heating the feces material should also be considered. It is essential to note that the actual heat losses in the system would affect the heat supply calculation significantly, and calculations should include these aspects for an accurate evaluation of the process.

Key concepts

Energy from Waste

The concept of energy from waste involves capturing the potential energy contained within waste materials and converting it into usable power. In the context of the exercise, fecal waste from a community is being considered as a source of energy through combustion. However, before the energy can be extracted efficiently, the moist feces must be dried to increase its dry matter content.

Transforming waste into energy not only provides a source of renewable power but also addresses waste disposal issues. By using sewage waste, communities can generate electricity, heat, or fuel, thereby reducing the reliance on fossil fuels and cutting down on the volume of waste sent to landfills.

Thermal Conversion Processes

Thermal conversion processes, such as combustion, pyrolysis, and gasification, are methods used to extract energy from materials by subjecting them to high temperatures. In this exercise, combustion requires the feces to have an 80 wt% dry matter content for optimal energy production.

The drying of the wet biomass, a key step in the process, involves the removal of water, which requires significant energy input. Once dried to the appropriate level, the feces can be burned efficiently to release energy. Such thermal processes are common in waste-to-energy facilities, where they convert municipal and industrial waste into electricity and/or heat.

Water Evaporation Energy

Calculating the energy necessary for water evaporation—part of the drying process—is critical in optimizing energy production from waste. This exercise illustrates how to calculate the energy required to heat and evaporate the water content in feces.

The energy calculations are straightforward: you need to consider the specific heat capacity of water to heat it to the evaporation point and the latent heat of vaporization for the actual phase change from liquid to gas. In this scenario, it's essential to realize that the energy input for water removal would have to be less than the energy gained from the waste to ensure a net positive energy outcome, making the process viable as a renewable energy source.

Sustainable Energy Sources

Sustainable energy sources are those that are replenished naturally and have a low environmental impact. Biomass energy production, as exemplified by this exercise, falls into this category. The concept is predicated on the principle of sustainability, meaning the system can maintain itself long-term without depleting resources or harming the environment.

Extracting energy from community waste is sustainable since it repurposes an otherwise troublesome byproduct. Moreover, by enhancing the process's efficiency – perhaps by utilizing urine separation and optimal drying – the overall sustainability of such energy production can be significantly improved. This aligns with global efforts to shift away from unsustainable fossil fuels towards cleaner, renewable sources of energy.