Sustainability Assessment of Wastewater Treatment Plants

Başak Kiliç Taşeli

### Abstract

It is thought that this chapter will make a significant contribution to the literature or at least will fill the space on the wastewater treatment plant's effect on climate change. It demonstrates the potential climate change impact of a sequential batch reactor (SBR) and constructed wetland on treating domestic wastewater by giving methods for calculation of their greenhouse gas emissions in terms of N2O and CH4. Are wastewater treatment plants sustainable? What aspects determine sustainability? Do tertiary wastewater treatment plants and constructed wetlands (CWs) have less global warming potential (CO2 emissions) and less energy use than conventional treatment? In accordance with the literature, greenhouse gas calculations of this study showed that CWs and SBR WWTPs do not contribute to global warming negatively.

Keywords: wastewater treatment, sequential batch reactor, greenhouse gas, constructed wetlands, methane, nitrous oxide, sustainability

### 1. Introduction

Wastewater treatment plants are generally capable of reaching hygienic and environmental standards; however, these were not designed for zero discharge principle in which nutrients, organic matter, and water are recycled and nutrient, organic and water cycles are closed. Are wastewater treatment plants (WWTPs) sustainable? What aspects determine sustainability? Do tertiary wastewater treatment plants and constructed wetlands (CWs) have less global warming potential (CO2 emissions) and less energy use than conventional treatment?

Since sequential batch reactor (SBR) system sequentially removes carbon, nitrogen, and phosphorous in a single reactor by maintaining anoxic and aerobic stages, it recently has attracted a great deal of interest. High nitrogen and phosphorus removal are achieved by a series of steps, namely, fill, react, settle, draw, and idle steps, as shown in Figure 1. Denitrification occurs at the beginning of the fill step taking usually 25% of the total cycling time where raw wastewater is added to the reactor. The step taking up 35% of the total cycle time is called react step where the reactions were finalized. The main purpose of the third step (settle) is to allow solid separation and provide a supernatant ready to be discharged as effluent. The purpose of the fourth step (draw step) ranging from 5 to 30% of the total cycle time is to remove clarified treated water from the reactor. The purpose of last step, "idle," is to provide time for one reactor to complete its fill cycle before switching to another unit.

nitrous oxide is a significant greenhouse gas with a lifetime of 114 years with a 298 fold stronger effect of global warming than carbon dioxide and is also responsible

In SBR processes, ammonium is transformed into N2 gas via nitrification and denitrification. N2O is generated as a by-product or an intermediate due to insufficient oxygen during nitrification in the aeration step and due to insufficient carbon during denitrification in settling and decanting steps [6]. Wastewater treatment facilities are anthropogenic sources of N2O to the atmosphere, taking account of 3.2–10% of the total emission [7]. Practically, only methane and nitrous oxide are calculated since carbon that is present in wastewater is biogenic and it is assumed that it is returning the carbon to the atmosphere as CO2 representing no net flux to

Based on field measurements, the maximum methane flux occurred in sludge

The greenhouse gas emissions measured in N2O, CO2, and CH4 for horizontal

The CO2, CH4, N2, and N2O fluxes in both horizontal and vertical subsurface flow constructed wetlands in Estonia were measured and reported that the global influence of constructed wetlands is not significant, that is, even if all global domestic wastewater were treated by constructed wetlands, the emitted GHG would be <1% of total anthropogenic emissions [15]. They also reported the averaged experimental

The following section will represent CH4, N2O, and CO2 emission calculating

2. Emission calculating principles for wastewater treatment plants

Estimation of organically degradable material in domestic wastewater, estimation of methane emission factor (EF) for domestic wastewater, and estimation of CH4 emissions from domestic wastewater are steps for calculating

[13]. Moreover, vertical flow constructed wetlands (VFCWs) had significantly higher areal gaseous emissions than HFCWs, and gas emissions were correlated to temperature, substrate supply (influent N and C concentrations), and degree of oxidation in the wetland [14]. The quantity and impact of CH4 and N2O are important since CH4 has 25 times and N2O has 298 times the global warming potential of

flow constructed wetlands (HFCWs) were 3, 1400 and 5 mg/m2

data of 788.33 mg CO2/m2 h, 4 mg CH4/m2 h, and 0.79 mg N2O/m2 h.

principles for both SBR and CW WWTPs.

2.1 Methane (CH4) emission calculating principles

unit [9]. Methane is produced by methanogens due to low O2 and nitrate/nitrite concentration during the anaerobic and anoxic processes. In the same direction, more than 50% of global methane emissions are related to human-related activities like landfill, wastewater treatment, agriculture, and certain industrial process [10]. Are constructed wetlands sustainable? It is reported that constructed wetlands have less global warming potential (CO2 emissions) and less energy use than conventional treatment [11]. Wetlands also reduced aquatic toxicity and eutrophication compared to conventional activated sludge wastewater treatment [12]. A sustainable solution means minimized costs; minimized energy use; minimized land area required; minimized loss of nutrients; minimized waste production; maximized products like clean water, biogas, biomass, fertilizers, and compost; and maximized qualitative sustainability indicators like social acceptance, institutional requirements, etc. But it is not always possible to design a wastewater treatment that minimizes cost, energy use, and land area, while maximizing performance.

/d, and CH4 emission occurred in every processing

/d, respectively

for ozone depletion in the stratosphere [5].

DOI: http://dx.doi.org/10.5772/intechopen.88338

Sustainability Assessment of Wastewater Treatment Plants

the system [8].

CO2 [4].

CH4 emissions.

85

screw conveyor with 823 g/m2

#### Figure 1. Operation sequence for sequential batch reactor [1].

In wastewater treatment technologies like activated sludge, membrane methods are not feasible enough for widespread application in rural areas [2]. Constructed wetlands however are attracting great concern due to lower cost, easy operation, and less maintenance requirements as a reasonable option for treating wastewater in rural areas. They are designed and constructed to mimic natural wetland systems for removing contaminants which are basically composed of vegetation, substrates, soils, microorganisms, and water, utilizing complex processes involving physical, chemical, and biological mechanisms (e.g., sedimentation, filtration, precipitation, volatilization, adsorption, plant uptake, and various microbial processes) [3].

While the treatment performance of CWs is critically dependent on the optimal operating parameters (water depth, hydraulic retention time and load, feeding mode and design of setups, etc.) which could result in variations in the removal efficiency of contaminants, plant species and media types are crucial influencing factors for the treatment in CWs as they are considered to be the main biological component of CWs. Emergent, submerged, floating-leaved, and free-floating plants are commonly planted among 150 macrophyte species. The most common used emergent species reported are Phragmites spp. (Poaceae),Typha spp. (Typhaceae), Scirpus spp. (Cyperaceae), Iris spp. (Iridaceae), Juncus spp. (Juncaceae), and Eleocharis spp. (Spikerush) [3].

The greenhouse effect of major greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) all produced in wastewater treatment operations, is weighted by their global warming potentials (GWP). Over a period of 100 years, 1 ton of methane and nitrous oxide will have a warming effect equivalent to 25 and 298 ton of CO2, respectively [4]. In the same direction, it is stated that

nitrous oxide is a significant greenhouse gas with a lifetime of 114 years with a 298 fold stronger effect of global warming than carbon dioxide and is also responsible for ozone depletion in the stratosphere [5].

In SBR processes, ammonium is transformed into N2 gas via nitrification and denitrification. N2O is generated as a by-product or an intermediate due to insufficient oxygen during nitrification in the aeration step and due to insufficient carbon during denitrification in settling and decanting steps [6]. Wastewater treatment facilities are anthropogenic sources of N2O to the atmosphere, taking account of 3.2–10% of the total emission [7]. Practically, only methane and nitrous oxide are calculated since carbon that is present in wastewater is biogenic and it is assumed that it is returning the carbon to the atmosphere as CO2 representing no net flux to the system [8].

Based on field measurements, the maximum methane flux occurred in sludge screw conveyor with 823 g/m2 /d, and CH4 emission occurred in every processing unit [9]. Methane is produced by methanogens due to low O2 and nitrate/nitrite concentration during the anaerobic and anoxic processes. In the same direction, more than 50% of global methane emissions are related to human-related activities like landfill, wastewater treatment, agriculture, and certain industrial process [10].

Are constructed wetlands sustainable? It is reported that constructed wetlands have less global warming potential (CO2 emissions) and less energy use than conventional treatment [11]. Wetlands also reduced aquatic toxicity and eutrophication compared to conventional activated sludge wastewater treatment [12]. A sustainable solution means minimized costs; minimized energy use; minimized land area required; minimized loss of nutrients; minimized waste production; maximized products like clean water, biogas, biomass, fertilizers, and compost; and maximized qualitative sustainability indicators like social acceptance, institutional requirements, etc. But it is not always possible to design a wastewater treatment that minimizes cost, energy use, and land area, while maximizing performance.

The greenhouse gas emissions measured in N2O, CO2, and CH4 for horizontal flow constructed wetlands (HFCWs) were 3, 1400 and 5 mg/m2 /d, respectively [13]. Moreover, vertical flow constructed wetlands (VFCWs) had significantly higher areal gaseous emissions than HFCWs, and gas emissions were correlated to temperature, substrate supply (influent N and C concentrations), and degree of oxidation in the wetland [14]. The quantity and impact of CH4 and N2O are important since CH4 has 25 times and N2O has 298 times the global warming potential of CO2 [4].

The CO2, CH4, N2, and N2O fluxes in both horizontal and vertical subsurface flow constructed wetlands in Estonia were measured and reported that the global influence of constructed wetlands is not significant, that is, even if all global domestic wastewater were treated by constructed wetlands, the emitted GHG would be <1% of total anthropogenic emissions [15]. They also reported the averaged experimental data of 788.33 mg CO2/m2 h, 4 mg CH4/m2 h, and 0.79 mg N2O/m2 h.

The following section will represent CH4, N2O, and CO2 emission calculating principles for both SBR and CW WWTPs.

### 2. Emission calculating principles for wastewater treatment plants
