Multi-stage evaporation process – Low energy costs for leachate treatment

For years, scientists and engineers have recognized the efficiency of multi-stage, flash evaporation. In very general terms, it takes approximately 700 kWh/m³ of heat energy to cause water to evaporate. This very high number is based on a single stage situation which would be analogous to a kettle on a stove. The Multiple Stage Flash process, when incorporating 15 stages (approximately), would only require 50 to 70 kWh/m³.

The unfortunate part of this dramatic improvement in efficiency is the fact that Multiple Stage Flash units are extremely difficult to operate under ideal conditions and virtually impossible to operate if there are any fluctuations in the heat energy or in the characteristics (temperature, concentration, flow rate) of the mother fluid. In reality, each stage operates at a different pressure and temperature.

The first stage would typically be 90° C operating at half an atmosphere of pressure. The last stage would typically be operating at 35° C at 1/20th or 1/25th of an atmosphere. Each progressive stage would operate at slightly lower pressure and temperature in order to achieve the very high efficiency of evaporation. Non ideal operating conditions will cause instability in the stages. When the system is not operating in a stable condition, no progress (separation of distillate from leachate) is achieved (See diagram).

The Atlantis AutoFlash process incorporates a static, self-regulating device separating each stage from the next. This proprietary device allows the benefit of evaporation to be available to the wastewater industry with absolutely no operational difficulties.

Even more important is the fact that the low-grade, reject, waste heat can be used for driving the evaporation process. This quality of waste heat is so low, it is regarded as being available free of cost.

Further, the AutoFlash system is well applied on a landfill application generating electricity where the reject, water jacket heat from the internal combustion engine is utilized to efficiently and effectively drive the AutoFlash process. This energy source is the only heat required.

With the exception of volatile organic compounds (VOCs), the manufacturer claims that the system effectively removes 99.5% of ammonia, salts, heavy metals, BOD and COD.

Based on over 20 years of experience, the AutoFlash system requires approximately 1 1/2 man hours per day, based on a five day work week, for the fulfillment of all operating and maintenance requirements. The simplicity of operating and control makes AutoFlash ideally suited for remote SCADA monitoring from other manned locations.

The manufacturer states that, based upon offerings made in recent weeks, the total amortized capital cost including operating & maintenance is less than 2.5 cents/gallon ($6.60/m³ Cdn).

Some important design parameters to consider:

• On leachate applications, the unit energy consumption for an AutoFlash system generally runs between 180 to 200 kWh/m³ of leachate. This, of course, should be waste heat or heat from directly firing a boiler with landfill gas.
• Energy consumption for all other utilities for an AutoFlash Advanced Leachate Treatment System would run between 8 and 12kWh/m³. These other utilities would include: vacuum pumps, transfer pumps, cooling towers and other equipment driven by electric motors. These components could be supplied internally if the generation of electricity was, in fact, part of the specific project.

There are several AutoFlash applications to consider:

1. The simple volumetric reduction of leachate (96%+) through utilization of AutoFlash, resulting in lower trucking costs.
2. Volumetric reduction of leachate through utilization of AutoFlash, and recycling the concentrated leachate through the landfill mass.
3. Co-generation, whereby the landfill gas drives an engine to create electricity, and the waste heat from the engine drives the AutoFlash to volumetrically reduce the leachate quantities. The ultimate disposal of the concentrated leachate could either be through recycling or alternate methods, including incineration.
4. The waste heat from whatever source could attract an AutoFlash hazardous waste treatment facility (volumetric reduction) due to the low costs of evaporation, and the ease with which AutoFlash can be stopped and started.
5. AutoFlash technology has universal applications in the chemical process field, food industry as well as the desalination of seawater.

The Atlantis AutoFlash was first successfully used on a full-scale basis in 1979. Several installations have utilized landfill gas. According to the manufacturer, the history of operating experiences in many countries has demonstrated the reliability of this unique, cost-effective, and proprietary system.