A novel concept to integrate energy recovery into potable water reuse treatment schemes

Potable water reuse applications can provide a safe and sustainable water supply where conventional freshwater resources are limited. The objectives of this study were fourfold: (i) to analyse existing potable water reuse applications regarding operational characteristics and energy demands, (ii) to determine the theoretical energy potential of wastewater and identify opportunities for energy recovery, (iii) to de ﬁ ne design requirements for potable water reuse schemes that integrate energy recovery and (iv) to propose strategies for more energy ef ﬁ cient potable water reuse schemes. Existing potable water reuse schemes commonly utilize conventional wastewater treatment processes including biological nutrient removal followed by advanced water treatment processes. While meeting high product water quality, these treatment schemes are characterized by relatively high speci ﬁ c energy demands (1.18 kWh/m 3 ). Given that the theoretical energy potential of municipal wastewater is approximately two times higher (2.52 kWh/m 3 ), opportunities exist to integrate energy recovery strategies. We propose three alternative potable water reuse schemes that integrate energy recovery from carbon via methane and nitrogen via either the coupled aerobic – anoxic nitrous decomposition operation process or partial nitritation/anammox. Compared to conventional potable water reuse schemes, the energy requirements of these schemes can be reduced by 7 – 29% and the overall energy balance by 38 – 80%.


INTRODUCTION
Population growth and demographic shifts, climate change impacts, uneven distribution of freshwater resources, water scarcity and emerging water quality issues are becoming more and more pressing worldwide (Mekonnen & Hoekstra ; United Nations ). Various options exist to provide sufficient water supplies locally. These options depend on local availability and other site-specific factors and range from conventional surface water supplies (from rivers and lakes) or groundwater to brackish water or seawater desalination and importing water from other watersheds.
Depending on the type of water source, the required energy demands for treatment (no water conveyance and distribution included) are typically 0.05-0.37 kWh/m 3 for conventional water supply from surface water, 0.19-0.58 kWh/m 3 for conventional water supply from groundwater, 0.26-2.6 kWh/m 3 for brackish water desalination Cooley & Wilkinson ; NRC ). However, these potable water reuse schemes represent a more cost-efficient option to augment local water supplies than brackish water or seawater desalination. Nevertheless, compared to conventional water supplies their specific energy demands are still significant.
Potable water reuse has been practised for more than 50 years and provides reliable and safe drinking water. Significant improvements in individual treatment processes and improved water quality monitoring have resulted in an increased confidence in potable water reuse practices worldwide (Drewes & Khan ; Schimmoller et al. ).
However, the widespread implementation of potable water reuse schemes is hindered by the lack of public confidence and regulatory uncertainty but also by high energy consumption and subsequent high operational and maintenance costs (O&M). With increasingly more stringent water quality requirements, it is expected that additional treatment steps for the removal of emerging microbial and chemical contaminants will be required that will further increase the energy demand of water treatment systems in the future.
In the past, significant process optimizations have resulted in improved energy efficiency of the overall treatment scheme (mainly by improved aeration systems for activated sludge processes, employment of energy recovery devices for high-pressure membranes and by utilization of anaerobic treatment processes). However, the current design philosophy of water reclamation facilities is still focused on initial biological carbon and nutrient removal followed by advanced treatment with little attention to simultaneous energy recovery. A new paradigm has emerged that is shifting the perception of wastewater as a disposal issue to an opportunity to continuously recover resources including nutrients, energy, heat and water (McCarty et al. Untreated municipal wastewater contains potential, thermal and chemical bound energy. The potential energy (E pot ) of wastewater is proportional to its elevation and can be calculated with: where m is the water quantity/mass (kg), g the gravitational acceleration of the earth (9.81 m/s 2 ) and h the elevation (m).
In theory, potential energy can be utilized in the sewerage system or the influent and/or effluent of wastewater treatment plants (WWTPs). In many settings, this energy is not considered a significant energy source due to gravitydriven conveyance systems and dependency on the local topography.
The amount of recoverable thermal energy (E therm ) can be calculated by: where c p is the specific heat capacity of water (4.18 kJ/(kg × K)), ΔT m the temperature gradient (K) and m the water quantity/ mass (kg As well as thermal energy, electrical energy can be generated from the chemical energy potential. This is the preferable option in terms of subsequent energy utilization as it is more versatile and can be transported almost without loss. The chemical energy potential of waste streams is commonly assessed by the chemical oxygen demand (COD).
The energy content per g COD can be determined using the overall enthalpy (ΔH of products -ΔH of reactants) expressed in the following reaction, assuming methane (CH 4 ) is the organic substrate:

EXISTING POTABLE WATER REUSE SCHEMES
Potable water reuse is being practiced worldwide using a broad variety of treatment process configurations. This variety is due to a wide range of possible treatment options and different regulatory requirements. The trend to meet more stringent water quality criteria over the last 20 years has resulted in incremental improvements of unit processes in potable water reuse treatment train (TT) design.

Treatment train characteristics
Existing potable water reuse schemes can be classified into membrane-based or non-membrane-based TTs and treatment schemes with or without an environmental buffer

MUNICIPAL WASTEWATER AS ENERGY AND NUTRIENT SOURCE
Municipal raw wastewater contains a considerable chemical energy potential (see Introduction). Thus, the energy and nutrient content of municipal raw wastewater was characterized through weekly measurements over a period of one year at the WWTP at Garching, Germany. Wastewater influent was collected after a 4 mm drum screen. Average values were determined as follows: flow rate 4,644 ± 724 m 3 /d, temperature 15.3 ± 2.4 C, pH 8.24 ± 0.06 and electrical conductivity 1,438 ± 241 μS. The theoretical energy potential has been determined to vary between 1.9 and 3.14 kWh/m 3 (mean 2.52 kWh/m 3 ) (Figure 3 and Table SI 1 in the supplementary material, available with the online version of this paper).

ENERGY RECOVERY PLATFORMS (FROM CARBON AND NITROGEN)
Raw wastewater represents a high theoretical energy poten- where ammonia (NH 4 þ ) is oxidized to nitrite (NO 2 À ) and nitrite is oxidized to nitrate (NO 3 À ). The process is stoichiometrically expressed as (Schmidt et al. ):

ALTERNATIVE POTABLE WATER REUSE SCHEMES
This section provides the design requirements that integrate energy recovery and explains and compares the alternative TTs with each other and with the defined benchmark for potable water reuse schemes. The overall energy balance, the GHG potential, the effluent water quality and the process stability are considered.

Design requirements
The design of alternative potable water reuse schemes requires a thorough understanding of (i) source water characteristics, (ii) regulatory and water quality requirements, (iii) maintaining proper performance of treatment processes and their combinations, and (iv) storage and blending requirements. Maintaining system reliability (by establishing redundancy, robustness and resilience) is a very important design element for potable water reuse      Figure 5(b)). Considering both energy recovery and GHG emissions, TT II is the most sustainable potable water reuse scheme with a net energy balance of 0.32 kWh/m 3 and 0.45 kg CO 2 e/kWh based on the assumptions of this analysis. However, additional research is needed to demonstrate feasibility, reliability and water quality of the proposed alternative water treatment as well as the CANDO implementation in a main-stream application.
The main requirement of all potable water reuse schemes is the reliable and continuous generation of highquality water in compliance with drinking water regulations.
A wide range of naturally occurring and anthropogenic trace organic and inorganic contaminants, residual nutrients, total dissolved solids, residual heavy metals and pathogens  Table 2).

CONCLUSIONS
Environmental sustainability is one of the most critical challenges in contemporary water and wastewater management.
The aim of technologies employed to treat water and wastewater should not only be to remove relevant contaminants, but also to achieve high energy efficiency by recovering useful resources from wastewater. The theoretical energy content in raw wastewater is 2.52 kWh/m 3 . Conventional potable water reuse schemes (CAS þ AWT) currently require an average of 1.18 kWh/m 3 . Integrating anaerobic treatment (e.g. AnMBR) and CANDO offers the opportunity to establish more energy efficient potable water reuse schemes with significant lower GHG emissions. However, more detailed investigations of these alternative treatment schemes are needed including a detailed technical feasibility study. In particular, the feasibility of the proposed physical treatment processes (microsieving, UF and RO) coupled with subsequent biological (BAF) and disinfection processes require detailed research to overcome operational issues, demonstrate long-term operation and guarantee final water quality for drinking water purposes.