Wastewater - increasing the stability

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Wastewater - increasing the stability

November 24, 2010 - 14:00
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BRUSSELS, Nov. 25, 2010 (RISI) -A reliable treatment performance for wastewater is a key business driver in today's world, and is a critical factor in minimizing the effect on other plant processes to take advantage of replacing fresh water with a stable process/clarified water source. However, wastewater treatment is a dynamic process with variable treatment efficiency. This variability can be caused by mechanical conditions, operational practices, or simply because of the nature of the waste treatment plant (variability of the contaminants in the stream), and these solids/liquids separation provide individual challenges at each location. One piece of equipment may also be similar from one application to another, but problems due to the unique chemistry involved may be different. The bottom line is that it is necessary to minimize variability of the results even the though the conditions may not be optimal.

The cost of chemical treatment in a wastewater treatment plant is easy to identify. But this is only part of the total cost of operation (TCO) where the primary goal is to reach high quality water/sludge at the lowest cost by minimizing the overall operating cost of the facility. There are several ways to accomplish this, as evidenced though the following examples:

  • Efficiency improvement of treatment processes
  • Increased quality of water/sludge output
  • Minimized process variability (by absorbing the impact of dynamic nature of the process)
  • Replacing fresh water usage with process (clarified/filtered) water where applicable.

Additionally, wastewater treatment can typically be divided in three key treatment phases:

Primary:Coagulation, clarification or flotation processes;

Secondary:Normally includes a biological treatment;

Tertiary:To provide final and fine tune treatment.

In the examples cited throughout, due to the unique treatments of each facility and the ability to evaluate, optimize and troubleshoot any wastewater treatment process, the focus will be addressing primary treatments.

Fundamentals of injection technology

There are a number of methods utilized for chemical delivery into process pipes, including direct hose/pipe connection (i.e. T-jet type of connection), injection stub, drilled quill and several newer more novel engineered methods. Each of these methods has inherent advantages and disadvantages and requires certain compromises and technical tradeoffs in order to be successfully implemented.

PARETO Mixing Technology has been specifically designed to reduce the inefficiencies and challenges that are inherent for these methods. This technology was developed by Nalco and is based on computational fluid dynamics (CFD) modeling. It is now an established best practice for flocculant addition across the company. While PARETO technology is applicable to all chemical additions, it is particularly effective in optimizing flocculant dosing due to the rapid and irreversible nature of the polymer adsorption process onto system additives. PARETO technology is a patented, engineered system that enhances chemical performance by optimizing the injection of the chemical additives into industrial process streams. The technology allows for a more uniform additive distribution to be achieved in a shorter time compared to traditional injection systems.

The single jet injection system, also called a "Tee Mixer", is commonly used for many chemical applications. Chemical injection performance is evaluated by measuring the uniformity of chemical concentration at different locations, downstream from the injection point.

For many single-jet systems, complete mixing does not occur. Hence, this results in a high, localized chemical concentration or floc formation, resulting not only in reduced chemical efficacy, but variability in floc formation. This may have a detrimental effect on certain finished product attributes such as formation, porosity, moisture profiles as well as impact operational efficiency and or instability.

With PARETO technology simulation complete mixing occurs within two meters (7 ft) of injection. This eliminates many of the issues resulting from poor mixing as were just described, and allows for opportunities to move the injection point closer to the reaction chamber (headbox/solid-liquid separation device) and enhance flocculant performance.

The conical PARETO technology jet is discharged into the approach pipe in a manner that allows contact with the stream to treat (paper furnish, influent, sludge line) in the turbulent flow region of the pipe. The chemical solution thus mixes with the furnish components as quickly and as uniformly as possible. The chemical penetration affect results from the energy of the mass flow and pressure of the mixture.

After success in wet end optimization programs was evident, PARETO technology was expanded to other areas/applications in the locations where the technology was being used and where local teams comprised of customers and Nalco representatives, identified programs with a lack of or poor mixing.

The technology is now an incumbent solution for different applications with different types of chemical programs (Beta tests are currently being conducted within other Nalco industries to identify new applications). Some examples of the new applications are as follows:

  • Flocculants in dissolved air flotation units (DAF's)
  • Coagulants and flocculants in screw presses
  • Flocculants in gravity tables and centrifuges (sludge thickening)
  • Defoamers in pulp mill washers and paper processes.

Case study: 100% recycled board mill

Nalco had successfully installed PARETO Mixing Technology optimizers on the process side (RDF, retention/drainage/formation in a per machine) at a customer site. The offering effectively balanced the interrelated and dependent variables of hydrodynamics, chemistry and water management. Due to this success, the Nalco team at the mill and the mill's customer management decided to explore new opportunities in trying to integrate the process into the wastewater treatment side. The idea was the involvement of a new stage - improving the mixing between polymer (cationic latex high molecular weight) and influent, aiming for stability in dosage as well as ensuring that optimal chemical efficacy was achieved.

As a brief operational summary, there are two basic flotation processes: dispersed air flotation and dissolved air flotation. In dissolved air flotation, minute gas bubbles are formed by the precipitation of air from a supersaturated solution. Generally, the bubbles are a smaller size of 10 to 100 microns.

Polymers improve flotation by increasing the size of the particles in the waste. The coagulants neutralize the electrical charges surrounding the particles, allowing them to agglomerate. The flocculant bridges are the agglomerations forming a large structure for the absorption and trapping of the air bubbles. The most effective polyelectrolytes have either been moderate molecular weight coagulants or very high molecular weight cationic flocculants.

Mill overview: 100% recycled (domestic OCC)

DAF 1

  • Influent flow: 12,000 - 15,000 L/min (3,100 - 4,000 gal/min)
  • Influent concentration between 2,000 - 2,500 ppm
  • Effluent: 150 ppm max
  • Collects streams from entire mill.
Flocculant monthly consumption in DAF 1 after PARETO technology installation

DAF 2

  • Influent flow 4,000 L/min (1,000 gal/min)
  • Influent concentration of 300-400 ppm
  • Effluent: 100 ppm max
  • Collects white water stream from two paper machines.
Flocculant monthly consumption in DAF 2 after PARETO technology installation

The main goal was divided into two challenges:

Operational:Minimizing process variability.

  • Ability to absorb influent variation in WWTP
  • Improvement of DAF stability
  • Better control on fines removal.

Environmental:Reducing fresh water consumption.

  • Reduction of fresh water usage to meet specs
  • Use of higher quality clarified water in areas within the mill, which includes pulp dilution after high consistency refiner, and feed liquid in the optimizer.

Program design

In a conventional T-Jet injection system for these chemical aids, the flocculant is made down to a specific concentration in order to get the best inverted polymer. It is then diluted with fresh water prior to the feed point in order to obtain the best interaction with influent. In this case, the polymer was being inverted to a lower concentration and being fed without extra dilution water. Nalco was able to invert the polymer properly and add sufficient water to obtain a much better mixing. The main concept is that if someone is injecting the inverted polymer into a process stream pipeline, full distribution of the product stream at a desired distance downstream will be achieved. The key business drivers were DAF efficiency and total cost reduction.

PROGRAM RESULTS

I. Operational (DAF efficiency)

  • Less effluent variability
  • Improved solids removal with the lower flocculant dosage, as indicated in Fig. 1 and 2.

II. Environmental (stability of clarified water)

  • Used 75% less of the fresh water (100 m3/day or 9.25 MM gallons/yr)
  • Clarified water used in other processes such as pulp post dilution and feed liquid in the PARETO technology optimizer
  • Used to drain silo; was utilizing fresh water once/week, now only once/month
  • Saved more than 42 tons/yr (11,000 gallons/yr) of latex flocculant (less VOC)

III. Reduction of total cost of operation (TCO) and financial impact

  • Better control on forming table (runnability); paper sheet moisture uniform to press section, less down time, more production
  • About $100,000/yr in savings associated with less chemical usage and fresh water replaced by clarified water
  • Ability to keep critical properties in main grade (sack paper), such as porosity, by having fewer fines in the short loop.

Conclusion

There was a flocculant dose reduction, (17-40%) due to improvement in mixing efficiency, allowing the customer to handle 42 tons/yr less cationic flocculant (latex) and reduce the amount of VOC in the system. The customer was also able to remove fines from this post-dilution stream in order to have better stability in parameters, such as porosity without the need to drain white water in the process and replace it with fresh water.

PARETO Mixing Technology has also shown benefits in terms of on-machine efficiency (OME) gains recognized as a result of reductions in wet-web breaks caused by cleaner post-dilution stream. These improvements have resulted in improvements in finished product quality parameters. Maintenance of the PARETO system is simple and safe, and the technology optimizer is designed to be robust, durable, easy to install /disassemble for inspection and cleaning. Periodical checkups of the system are required in order to observe any changes in pressures and flows from the designed values. All of the variables described are balanced with water management to the extent that post-dilution water is optimized for source and demand. These components are balanced together to ensure that optimal delivery to the paper machine is achieved in terms of cost, operational efficiency and finished product quality.

Nalco reports environmental return on investment (eROI) values to customers to account for contributions in delivering both environmental performance and financial payback. Enhanced system performance through advanced chemical application yields better fiber recovery, and also delivers major reductions in the demand for water and energy. By reducing this demand, there is a direct impact upon the customer's environmental footprint and performance, and a major contribution to the sustainable development of paper mill operations.

Since the introduction of PARETO Mixing Technology in 2006, nearly 500 PARETO optimizer installations at 90 customer sites are expected to achieve savings after five years of operation (as of December 31, 2010) of more than 4.4 billion gallons of fresh water (approx. 16.7 million m3) and more than 18.8 million therms (>550 GWatt-Hr) of energy, resulting in significant financial savings for customers. These fresh water and energy savings are equivalent to 325,000 barrels of crude oil; 27,000 cars removed from roads; and 140,000 tons of CO2not being released to environment.