8.2.1 Inefficient Combustion of Fossil Fuels

The combustion of fossil fuels results in pollutant emissions that can damage human health and the environment on a number of levels. Most environmental concern has recently focused on global warming induced by the emission of so‐called “greenhouse” gases (GHGs), principally CO₂ emissions from fossil fuel combustion. However, combustion has very low efficiency. Table 8.2 lists the energy efficiency of a coal‐powered electrical station. To design the most energy‐efficient EEIS, the overall energy efficiency can be estimated using an IED approach. For example, an electric motor driving a pump that transports wastewater in WWTP comprises:

1. An electric motor (sizing and efficiency rating)
2. Motor controls (switching, speed or torque control)
3. Motor drive system (belts, gearboxes, etc.)
4. Pump
5. Pipework
6. Demand for fluid.

The efficiencies of these elements interact in complex ways. For example, if each element in the chain is improved in efficiency by 10%, then the overall level of energy use is , which results in a saving of 47%. Therefore, an IED approach could yield over 50% improvements. Corresponding to the energy efficiency improvement is the correlated reductions in negative environmental impacts. If the most efficient component is chosen for each part of a motor system, overall the efficiency of the whole system would be about seven times greater as shown in Table 8.3.

8.2.2 Challenges in China

In China, intensive fossil fuel consumption causes severe smog, which results in damage to human health and the environment. As a result, the Chinese government (2017) plans to reduce energy consumption as shown in Tables 8.4, 8.5, 8.6, 8.7, 8.8, and 8.9.

8.3.1 Thermodynamics Laws

Thermodynamic constraints and principles are applicable across both ecological and economic systems. Economic activity is entropic, while entropy increases as order decreases. Entropy leads the tendency of matter and energy to degrade or disperse into less useful forms during economic activity, although waste could be converted back to resources. Recycling of materials could close the loop of materials within society. The second law of thermodynamics requires that materials used in the economy tend to be used entropically. They get dissipated within the economic system unless reused or recovered. However, energy resource is usually degraded and difficult to be recycled depending upon the energy degradation rate. Typically, low‐entropy matter/energy comes in, while high‐entropy matter/energy goes out. The irreversibility of the real processes implies exergy destruction and waste discharged to the environment. Ecological economics starts from a recognition of the environmental, technological, individual, and social sources and support systems of productivity. Both natural and economic capitals should be maximally utilized. Table 8.10 is a brief history of sustainability by applying thermodynamic laws.

8.3.2 The First Thermodynamic Law

The first thermodynamic law defines conservation of materials and energy, which are not to be created or destroyed. Pearce and Turner (1990) analyzed the production process waste in the environment–economy interaction. The processing of resources creates waste W_R, as with overburden tips at coal mines; production creates waste W_P in the form of industrial effluent and air pollution and solid waste; final consumers create waste W_C by generating sewage and municipal solid wastes. The amount of waste in any period equals the amount of natural resources used, R:

(8.1)

The first law of thermodynamics states that energy and matter cannot be created or destroyed, but only transformed. Due to insatiable human demand, mining depletes natural resource, while consumers discard waste into the environment. As a result, mounting tasks are in front of the new generation of SEE designers to sustainably manage air, water, and solid wastes produced by modern societies.

8.3.3 The Second Thermodynamic Law

The second thermodynamic law states that work input into a system can be fully converted to heat and internal energy (via dissipative processes) without perfect efficiency. The maximum amount of work achievable in a given system with different energy sources in reference to its surrounding is referred as exergy. Exergy is the available energy for conversion from a donating source (or reservoir) with reference to a specified datum under ambient environmental conditions. It predicts the theoretical maximum energy available. Usually, high exergy degrades to low exergy continuously and irrevocably.

8.3.3.2 Enthalpy

Enthalpy is the heat content of gases, which depends not only on the internal energy (U) but also on the pressure (P) and volume (V). Thermodynamically, enthalpy is defined as

(8.2)

where

• H = enthalpy (kJ)
• U = internal energy (kJ)
• P = pressure (kPa)
• V = volume (m³)

The reaction heat is the enthalpy change of a reaction:

If there is no pressure change, then this is the heat absorbed:

8.3.3.3 Conservation of Energy

For nonbiodegradable materials in wastewater such as cellular fiber from papers, incineration could be the best process to recover heat energy (Figure 8.2). However, for a thermal power plant, about one‐fourth or one‐third of exergy is wasted due to the inefficiency of mixing and heat loss. On the other hand, chemical efficiency is usually much higher at 94–97%. For this reason, a fuel cell is a much more efficient electricity generation process than a thermal power plant.

8.4.1 Physical Framework by Thermodynamics

Energy, entropy, and exergy have been applied in environmental sustainability. Energy transformations within society have been viewed as resource depletion and environmental degradation by ecology, economics, and engineering.

Figure 8.3 shows that there is a trade‐off between capital cost, which is referred to as embodied energy, and OM cost.

Figure 8.4 plots the energy consumption against equipment size. There is a minimal value associated with the total energy requirement at a specific equipment size, which is also referred to as capital cost.

Hammond and Winnett (2009) argued that although the thermodynamic (or exergetic) improvement potential is around 80%, only about 50% of energy currently used could be saved by technical means, and when economic barriers are taken into account, this is reduced to perhaps some 30%. Nevertheless, there is significant potential for improvement in energy efficiency as shown in Figure 8.5.

8.4.2 Exergy

Exergy is the maximum theoretical work that a system can do before being brought into equilibrium with its environment. It is not conserved due to irreversibility, friction, or heat loss due to temperature difference. Exergy represents a theoretical thermodynamic value, while energy efficiency reflects an actual system. It measures the thermodynamic efficiency of a system by reflecting the quality of an energy carrier. In other words, exergy is simply a measure of the maximum theoretical useful work that is obtainable from a thermal system (as it is brought into equilibrium with its surrounding environment). Therefore, exergy analysis should be employed as a tool among several quantitative approaches to study energy systems, in addition to the traditional first law energy analysis:

(8.7)

where

• E = exergy
• U = internal energy
• P = pressure
• V = volume
• T = thermodynamic temperature
• S = entropy
• ΔKE = change in kinetic energy
• ΔPE = change in potential energy
• Subscript “_o” = reference environmental state

When a system is not moving nor has potential energy, it can be simplified as

(8.8)

Material resources are viewed as having chemical exergy, because of their disequilibrium with surrounding conditions. An increase in process efficiency will reduce the amount of waste and exergy degradation. Thus, exergy is seen as a measure of the value of the material, whereas process wastes have a potential to cause adverse changes in the environment. The exergy can be used as a potential tool for resource and/or emission accounting.

8.5.1 Unit Energy Consumption Values in WTP

For energy auditing in water treatment plants, typical unit value of energy consumption for each unit process of a WTP is very important. Table 8.11 compiles these unit values for WTP from Howe et al. (2012). It could be used as a benchmark in conducting energy auditing of any WTP.

8.5.2 Unit Energy Consumption Values in WWTP

For WWTPs, unit energy consumption data are extremely important when energy auditing is carried out in a WWTP. More importantly, the benchmark data for each unit process are critical to identify the energy gaps where energy efficiency could be improved by retrofitting the current WWTPs. Table 8.12 lists unit energy consumption data of each unit process of WWTP.

In the net zero guideline for WWTP, best practice values are significantly less than the typical value among 24 WWTPs surveyed in New York (Trarallo et al., 2015), so there is great potential to improve the performance of energy inefficient processes so that WWTPs can be retrofitted to be energy positive. In the last decade, major research efforts have been carried out all over the world such as in Singapore and Beijing, China (Gans et al., 2007) (Table 8.13). It shows that the overall energy efficiencies of WWTPs in Jurong and Gaobeidian are 35.9 and 31%, respectively. Since aeration and dewatering are the most energy‐intensive processes, typical and best practice values in terms of per kg pollutant removed, and unit energy consumptions in WWTPs all over the world have been compared in Table 8.14.

In the United States, the energy consumption data of 24 WWTPs in New York were reported (Tarallo et al., 2015). Here, technical benchmarks are statistically analyzed. Table 8.15 compares typical values and best practice values in terms of per kg pollutant removed and unit energy consumption in 24 WWTPs in the state of New York.

According to these data, the following histogram and cumulative plots are produced to establish the benchmark unit energy consumption values at percentiles of 10, 25, 50, and 90 to facilitate energy auditing and identify opportunities to retrofit existing WWTPs (Figures 8.6, 8.7, 8.8, 8.9, 8.10, 8.11, 8.12, 8.13, 8.14, 8.15, 8.16, 8.17, 8.18, 8.19, 8.20, and 8.21).

To provide quantitative unit energy consumption benchmark data, Table 8.15 listed values at normal and best practice of the 24 WWTPs in New York in terms of MJ/MG for normalized 10 MGD WWTP. In terms of unit energy consumption, influent pump station, biological treatment, and sludge handling are three major components. For each unit process, best minimal values are significantly less than the normal minimal values while the best maximal values are much smaller than that of the normal maximal values. Therefore, it could be used as an excellent technical benchmark in identifying energy efficiency project for retrofitting purpose of any WWTP. To visualize these data, histogram and cumulative curves for each unit process would help SEE designer to identify feasible technical targets in retrofitting projects at different quartiles in terms of the unit energy consumption performance. For example, 50% of typical bar screen and grit removal would consume 13.5 MJ/MG as shown in Figure 8.8 for side stream pump. If the WWTP is to be retrofit to achieve 10% in terms of unit energy consumption, the unit energy consumption for bar screen and grit removal should be less than 9 MJ/MG as shown in Figure 8.8.

8.6.1 Flow in Pipe

During the 1880s, an Irish engineer, Osborne Reynolds, directed tests by outwardly checking stream designs in glass tubes with color‐infused liquids. His investigation resulted in a common dimensionless ratio, the Reynolds number (Re). The Re is the ratio between the inertia force and the viscous force and relates to the physical and geometric properties of liquid thickness, speed, viscosity, and pipe diameter:

• Laminar (smooth) for Re < 2000
• Transitional for 2000 < Re < 4000
• Turbulent for Re > 4000

Moody diagram summarizes friction in pipe experimental results and establishes the relationship between the friction efficiency and Re and pipe relative roughness (ε/D). As a result, frictional headloss can be calculated by the Darcy–Weisbach (D–W) equation:

(8.9)

where

• h_L is the headloss (m)
• f is the friction coefficient
• L is the pipe length (m)
• D is the pipe diameter (m)
• V is velocity (m/s)
• G is the specific gravity (m/s²)

Water pumps are devices designed to convert mechanical energy into hydraulic energy, which is used to change the water level from a lower point to a higher point with a given discharge (flow rate) and pressure head.

Pumps can be classified as centrifugal pumps with dynamic or radial flow. Positive displacement pumps include rotary, reciprocating, and screw pumps. According to flow direction, centrifugal pumps can be classified as follows:

1. Radial flow, leaving the impeller in a radial direction.
2. Axial flow, leaving the propeller in an axial direction.
3. Mixed flow, leaving the impeller in a mix between the two directions.

Pump efficiency (η) is the ratio between pump output power divided by input power. Electrical motor efficiency is the ratio of output power divided by input power. Overall efficiency is the product of the efficiency of the pump and the efficiency of the motor. Over time, the pump characteristic curves will become steeper, and the operation point will get closer and closer to the y‐axis. Pump must provide sufficient pressure to overcome the total headloss of different ratios of flow rate (Q) and diameter (D) and deliver pressure at an equal or greater value than design standards:

(8.10)

where

• h_f is friction headloss (m)
• h_minor is the component headloss (m)

Pump power can be calculated as follows to quantify energy footprint of a pump:

(8.11)

where

• P is power (W)
• γ is specific weight of water (N/m³)
• Q is flow rate (m³/s)
• h_L is total headloss (m)

8.6.2 Pump Station

In pump station design, the net positive suction head available (NPSH_available) must be greater than the net positive suction head required (NPSH_required) to prevent cavitation of the pump. Otherwise the pressure will create implosions in the pump and decrease pump efficiency. The optimal guideline is to have at minimal 5–10% NPSH_available greater than NPSH_required. The basic equation to satisfy to prevent cavitation of pump is as follows:

(8.12)

where

• P_s/γ = suction head of the system
• P_v/γ = vapor pressure head of the pump

The best way to avoid cavitation would be to increase source pressure, raise the elevation of the source reservoir, and lower the elevation of the pump inlet. Otherwise, different pumps have to be selected to prevent cavitation of the pump.

For the pump and system to operate under steady conditions, pump total delivery head TDH_tot will have to equal the headloss in the system, as characterized by the system curve. That equality is satisfied under the conditions where the TDH_tot curve intersects the system curve. The value of Q at the operating point indicates the flow rate through both pumps and the piping. The head added by the original pump is the value of TDH_orig at that Q, and the head added by the pump which would have the value of TDH_boost at that Q.

8.7.1 Calculation Solar Energy

In designing zero energy residential houses, solar energy and biogas are two major sources. Therefore, how much energy is needed and how much solar energy could be harvested are the critical design problems that are illustrated by the following examples as provided in Solar Photovoltaic Systems Technical Training Manual developed by the UNESCO (Wade, 2003).

8.7.2 Solar‐powered WWTP

100% natural treatment of wastewater at a treatment plant by John Todd at Ecological Design known as the Eco‐Machine. The OCSL building is powered by solar and geothermal power. Unlike other WWTPs, the OCSL does not use chemicals to treat the water, but a combination of microorganisms, algae, plants, and gravel and sand filtration to clean sewage water and return clean drinkable water back to the aquifer. Environmental sustainability, green energy, and regenerative design are the key. All the wastewater from toilets, sinks, and showers on the Omega campus feeds into storage tanks that collect human waste. The wastewater is then sent to the Eco‐Machine and is fed to “microscopic algae, fungi, bacteria, plants, and snails.” The first stage is two 5000‐gal anoxic tanks located underground and microbial organisms use the wastewater digest carbon, ammonia, phosphorus, nitrogen, potassium, and many other substances in the water. Water then flows to four man‐made wetlands behind the OCSL building. The wetlands use microorganisms and native plants such as cocktails and bulrushes to reduce biochemical oxygen demand, odorous gases, and nutrients such as ammonia and phosphorus. As the wastewater flows through the wetlands, the microorganisms and plants are fed. After that, there is a 75% increase in water’s clarity and a 90% reduction in its odor. The aerated lagoons are divided into four cells, each 10 feet deep. Without soil, beautiful tropical plants thrive here. The plants live on metal racks and their roots extend up to 5 ft into the water. The roots of the plants act as a habitat for the organisms in the lagoon and are sustained by them. After the lagoons, the water moves back outdoors to a sand filter; after which it reaches the standards for treated water. The treated water irrigates Omega’s parking lot where it infiltrates into the groundwater table. As it trickles down to the aquifer that sits 250–300 ft below, campus water is further purified as it is withdrawn from deep wells that tap the aquifer; the water is used in sinks, toilets, and showers. Naturally the used water is reclaimed with the Eco‐Machine at OCSL, and the purified water is released back to the aquifer, where the process can begin again. This full‐circle process makes the Eco‐Machine incredible and is an inspiration to interconnectedness utilizing processes to meet human’s needs.

8.8.1 Questions

1. In a typical WWTP, how is high‐grade energy such as electricity degraded to mechanical, chemical, and biological energy? Please quantify these energy in kWh/m³.
2. What is embodied energy? Why there is an optimality between the embodied energy and operation energy consumption?
3. How do thermodynamic laws dictate energy recovery in WRRF? Please analyze how energy could be recovered from chemical and biological to electrical energy. What is the unit energy production rate in kWh/m³ during each major process?
4. In a typical WRRF, how much energy could be recovered in terms of theoretical energy contained in a typical wastewater sample of COD 500 mg/l and BOD 350 mg/l?
5. Where is the optimal point for operation cost versus equipment size?
6. What is the 10‐percentile unit energy benchmark for each process in WWTP?
7. How large is the roof area needed for a typical house in Miami, Florida, to achieve net zero energy design by using 40‐W solar panels?

8.8.2 Calculation

1. For three Miami‐Dade WWTPs of 300 MGD average flow rate, how much energy contained in average concentration of 330 mg/l COD and 223 mg/l BOD energy in theory if CO₂ is reduced to CH₄ or H₂, respectively? If only biological fraction is able to be reduced to CH₄, what are the maximal amount of energy that could be recovered in theory? If a typical theoretical value of energy content is about 1.75 kWh/m³, how is your estimate compared with this value?
2. If all the WWTPs are to recover energy BOD from anaerobic digestion and the biogas produced is to generate electricity with 25% efficiency through combustion engine or to generate electricity through fuel cell with 40% efficiency, what are the percentages of self‐sustaining electricity if the plant needs 0.39 kw/m³ to operate? Please list any assumptions that you would in terms of conversion efficiency.
3. If electricity costs $ 0.1/kWh, what would be the annual electricity bill for the operation of three WWTPs with the above energy recovery systems?

8.8.3 Project