The water and energy sectors are closely linked: it’s what is commonly called the water-energy nexus. In this series of articles, we will study the interactions between water and electricity and how changes in one sector are affecting the other. The development of renewable energies (RE) and lower power consumption water treatment technologies are not only deeply transforming the water sector, they are also increasing the possible synergies between the water and energy sectors.
“The water sector is defined as the sector that provides drinking water and wastewater treatment services to different end users (communities, businesses, industries). This sector can be divided into 4 sub-sectors following the water cycle: water collection, treatment, distribution, and overall management.”
The technologies used in water projects are very dependent on the scale, the raw water quality, the location and the end-use. In this series of articles, we will only consider small to medium scale, decentralised water treatment plants in developing countries as this is our core business at Impact Water Solutions (IWS).
In this first article of the “water-energy nexus” series (1/3), we want to assess energy consumption in the water sector:
When developing an integrated water project (drinking water and wastewater) for a client, the electricity consumption of the water collection, treatment, distribution, and management must all be considered and optimised. However, in most cases, the bulk of the electricity consumption occurs at the water treatment phase (for both wastewater and drinking water), so this is what we will focus on here.
Electricity consumption is a very important criteria to be considered when selecting a wastewater treatment plant (WWTP) technology, as it can strongly impact the operating costs (project feasibility) and environmental impact. For most WWTP technologies, at least 50% of the energy consumption comes from aeration (process of adding air to the sewage to promote biodegradation of the organic contaminants). Reduced electricity consumption is one of the factors that has been driving technological improvements in this field, however, there are still some important discrepancies between technologies. Electricity consumption of a WWTP is linked to many factors such as the quantity of water to be treated (there are “economies of scale”), quality of the input/output water, footprint or even operational skill level required.
Here is a rapid overview of different WWTP technologies:
- ASP (Activated Sludge Process): A multichamber reactor process that uses highly concentrated, suspended micro-organisms (to degrade organics and remove nutrients from wastewater) – this is the conventional and mostly centralised WWTP technology.
- SBR (Sequencing Batch Reactor): A single chamber reactor process that sequentially uses highly concentrated, suspended micro-organisms.
- MBR (Membrane Bioreactor): A multichamber reactor process that includes a final stage membrane chamber and uses highly concentrated, suspended micro-organisms.
- MABR (Membrane Aerated Biofilm Reactor): A single chamber reactor process that uses spirally wound membranes (for passive aeration) upon which highly concentrated micro-organisms grow.
- MBBR/FBBR (Moving Bed Biofilm Reactor/Fixed Bed Biofilm Reactor): A multichamber reactor process that uses highly concentrated micro-organisms, that either grow on moving or fixed media – also commonly known as SAF (Submerged Aerated Filters).
*Please note that other technologies exist, we have tried to give a general overview of the sector
Currently, various WWTP technologies with different energy consumptions are available on the market, yet they all reach low energy consumptions from a medium scale size. Technological improvements in this sector will most likely come from increasingly energy-efficient aeration processes.
DRINKING WATER TREATMENT
Drinking water treatment technologies (and their electricity consumption) highly depend on the quality of the water source. Indeed, surface water sources (rivers, lakes, reservoirs, etc.) will not require the same treatment as underground water or seawater.
Typical treatment methods for surface water include clarification/flocculation, basic filtration and chlorine or UV. Surface water generally requires treatment at low pressures; therefore, the energy consumptions of these technologies are typically low (<0.05 kWh/m3).
For water containing high mineral or heavy metal concentration, clarification and filtration methods are most commonly used as they are capable of effectively reducing the level of suspended solids (small particles including clay or microorganisms) in the water. Different types of filtrations exist including multi-media filtration (water flowing through different layers of media such as sand and gravel) and micro-/ultra-filtration (water flows through a semipermeable membrane of different pore sizes). However, all these methods don’t remove salt content, so when the source is brackish water or seawater, they are not able to fully produce drinking water. These different water treatment methods are sometimes complementary and are often used for the pre-treatment stages of Reverse Osmosis desalination.
For areas with only brackish water (BW) or seawater (SW) sources, desalination (i.e., removing salt content to produce drinking water) via Reverse Osmosis (RO) is the most common and efficient solution. In this process, water is forced at high pressure (overcoming osmotic pressure) through semi-permeable membranes to remove the salt content. Large-scale desalination plants have been installed for the past 40 years; however decentralised small to medium scale RO plants are only starting to gain momentum now. The main reason for this delay is the historically large energy consumption of RO treatment plants (over 6 kWh/m3 for SWRO). However, this energy consumption has been significantly reduced thanks to energy recovery devices (ERDs) such as pressure exchangers (these recover pressure from the brine waste stream for use in the feed stream of the RO process – see diagram below), as well as other configurational changes
Nowadays, large-scale centralised SWRO plants (production > 10,000 m3/day, TDS > 30 g/L) consume roughly 3-4 kWh/m3 water produced (including pre and post treatment); with 70% of this consumption linked to the primary RO process (i.e., roughly 2-3 kWh/m3 for the RO only)1. This energy consumption is, however, closely linked to feed/intake water quality (i.e., the greater the salt content the greater the pressure requirement), therefore producing drinking water from brackish water is much more efficient than from seawater (80% vs. 40% recovery rate).
Scheme of a typical SWRO desalination process. A pre-treated feed is supplied to the RO system with pressurization by HPP and BP, and the hydraulic pressure in the concentrate is recovered by ERD. RO: reverse osmosis. HPP: high-pressure pump. BP: booster pump. ERD: energy recovery device. 1
Small to medium scale seawater or brackish water desalination (SWRO / BWRO) is rapidly emerging as it enables off-grid clients to have a reliable and safe drinking water supply. These desalination plants are also particularly relevant as they obtain low electricity consumptions and can now integrate renewable energies (see next article of the series). For example, for the SWRO desalination plant IWS installed in Witsand (South Africa) in 2018, energy consumption is approximately 2.5 kWh/m3 (excluding pre and post treatment) for up to 300 m3/day. Overall, for desalination plants producing less than 1,000 m3/day, the electricity consumption is 2.5-4 kWh/m3 (SWRO) and 1-3 kWh/m3 (BWRO) for the RO only (excluding pre and post treatment).
In summary, progress in energy efficiency and overall low energy consumptions are making decentralised water desalination plants increasingly relevant.
At IWS we believe further optimisations are possible for such plants, including more efficient intake options such as beach wells (better intake water quality which leads to a reduction in pre-treatment requirements and hence lower energy consumption), easier integration into water grids and increased operating flexibility.
II. Water-Energy nexus (2/3): Optimising electricity supply
III. Water-Energy nexus (3/3): New synergies and opportunities
1 “A comprehensive review of energy consumption of seawater reverse osmosis desalination plants”, J. Kim, Applied Energy, 2019