Around the world, the water and wastewater sectors are embracing new technologies to improve supplies, reduce energy use and increase the environmental sustainability of this vital but finite resource. This article will look at some of the projects overseas and asks what the U.S. water industry can learn from them.
By Cameron Creech
The biggest driver for improved efficiencies in the water sector is obviously the need to provide sustainable water resources for a growing population in the face of potential
supply constraints such as climate change and conflicting demands. Globally, the United Nations estimates that some 3.6 billion people (almost half the world’s population) currently live in areas that are vulnerable to water scarcity and that almost 2 billion people could suffer water shortages by 2025.
Therefore, if water supply is to be fully sustainable, then energy efficiency is as important as water use efficiency. With this in mind, many techniques, which are already seen as industry best practice around the world, will need to become adopted more widely.
The potential of Water Resources Recovery Facilities to generate energy via anaerobic digestion (AD) is well understood, but in many cases, AD is used primarily for water treatment
rather than energy generation. Using the biogas produced for heat, power, or as renewable green gas will immediately bring both economic and environmental benefits.
Where thermal processing is used, heat recovery enables a resource that would otherwise go to waste to be used for purposes such as pre-heating items including process water, feedstock, digesters or evaporators to improve energy efficiency.
Heat exchangers represent the best way of recapturing heat from thermal processes, having a much lower energy requirement than tanks with heating jackets (up to half of that of some tank systems). In fact, a well-designed heat exchanger system could recover and reuse 40 percent of the heat produced by a wastewater AD plant.
One range proving popular with wastewater AD operators is a series of double tube heat exchangers. The inner tube is corrugated to ensure improved heat transfer performance and superior resistance against fouling, which also results in reduced maintenance periods. In addition, the tube in tube design permits the processing of fluids with particles without any tube blockage, making it particularly suited to wastewater, sewage and AD plants.
Improving the Value of Sludge and Digestate
A typical 5 million BTU wastewater AD plant can produce around 44,000 tons of liquid digestate each year, which creates significant economic and logistical challenges in terms of management, storage and transportation.
Generally, concentrated digestate and sludge is easier to manage. Using surplus or recovered heat to separate water from digestate by concentration can reduce the overall quantity of
digestate by as much as 80 percent, greatly lowering the associated storage and transport costs. A well-designed system will include measures to retain the valuable nutrients in the digestate, while the evaporated water can be condensed and returned to the front end of the AD process, reducing the amount of energy and water used by the plant. After concentration, the treated digestate dry solid content can be as high as 20 percent (often a four-fold improvement), making it much easier, and cheaper, to transport and handle.
Pasteurizing digestate and sludge to remove potential pathogens is a tried and tested technique around the world, allowing these valuable biofertilizers to be used on a wide range of soils and farming systems. One of the most energy-, and therefore cost-efficient methods to pasteurize digestate is a digestate pasteurization system (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurization is possible using surplus heat while allowing additional thermal regeneration levels of up to 60 percent. This saved heat can then be used for other processes, such as evaporation of the digestate to remove water.
Evaporation and cooling processes are commonplace in water and wastewater treatment, but evaporation can result in a high degree of material fouling on the inside of the equipment. Scraped-surface heat exchangers are designed to maintain thermal efficiency and remove fouling as it occurs.
Low temperature evaporation can be a very energy efficient method of water removal. Where process temperatures are 185° to 194°F, low temperature evaporation combines the use of a vacuum to reduce the boiling point of the liquid to be removed, together with traditional high temperature evaporators, based on heat exchanger technology. Where the temperature of the effluent or digestate falls below the necessary temperature, it can often be increased via heat exchangers, using surplus heat from heaters and CHP engines.
Combining systems into a multiple-effect evaporator allows larger quantities of water to be removed for the same initial heat input. Each evaporator is held at a lower pressure than the previous one: because the boiling temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next—only the first vessel requires an external source of heat, which can be taken from another process elsewhere or generated specifically for the purpose.
One common use of evaporation is in material recovery and waste valorization—the process of recovering value from waste materials. Solid-liquid mixtures are complex, and it is important that the first stage of resource recovery accurately evaluates the nature of the waste stream/s involved.
The evaporation and cooling steps are often involved in resource recovery result in a high degree of material fouling on the inside of the equipment, so scraped-surface heat exchangers are used to maintain thermal efficiency and remove fouling as it occurs.
Coupled with coolers and custom-designed crystallization tanks, the result is an efficient process that can work continuously without requiring scheduled downtime. Two evaporators are used to concentrate and remove pure water from the solution, which can be used elsewhere. The coolers and crystallizers produce solid crystals, and the remaining solution returns to the evaporation process. The system can be configured as a true Zero Liquid Discharge (ZLD) system, so no liquid waste remains after the process. This means that as recovering valuable minerals and salts, waste management costs are also reduced.
All of the techniques described are already technically feasible, and in many parts of the world are being implemented at trial or commercial scales. There may be a number of limiting factors, such as cost or levels of knowledge, which are slowing up their wider adoption on a local basis. However, if the industry is to continue to provide the world’s population with sustainable water sources into the future, then it will need to improve efficiency. In the medium term, techniques that today seem novel, such as desalination and direct water use, will have to become mainstream tools in delivering this ambition. | WA
Cameron Creech is General Manager of HRS Heat
Exchangers (Atlanta, GA), part of the HRS Group, which operates at the forefront of thermal technology, offering innovative heat transfer solutions worldwide across a diverse range of industries. With 40 years of experience in the water and wastewater sectors, specializing in the design and manufacture of an extensive range of turnkey systems and components, incorporating corrugated tubular and scraped surface heat exchanger technology, HRS units are compliant with global design and industry standards. HRS Heat Exchangers has been involved in project examples using all of the above technologies across the globe. For more information, call (770) 726 3540, e-mail [email protected] or visit www.hrs-heatexchangers.com.