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Laundry Wastewater Treatment

By Nathan Schiff, PhD
Associate Editor - Institutional

Introduction
Fifty years ago, it seemed OK to discharge untreated wastewater from industrial processes into rivers and lakes. Many believed that our supply of fresh water was unlimited and that it could somehow handle the waste. We also weren't as concerned with the long-term consequences of pollution, neither did we have the available technology to remedy the situation.
Virtually every Canadian municipality today, has adopted similar by-laws, specifying what can and can not be discharged into sanitary sewers and the permissible levels of each contaminant. Pressure from the municipalities and escalating sewer surcharges, which are applied when contaminant levels exceed the by-law maximums, are now making it attractive for commercial laundries to provide their own effective pretreatment program prior to discharging their wastewater.

What does wastewater from commercial laundries contain?
Fats, oils and greases (FOG) as well as total suspended solids (TSS), are the major impurities of concern. These impurities are derived from the soils which are removed from clothing as well as from the chemicals which are used. Laundry wastewater may also carry additional impurities, but for the purpose of this article we will address FOG and TSS, the most common contaminants.

Why are high levels of FOG so undesirable?
FOG and other organic chemicals are digested, to produce carbon dioxide and water, by sewage bacteria which feed upon them. In this process, these bacteria consume and deplete the dissolved oxygen in the water. If untreated wastewater were to be discharged directly into rivers and streams, it would also deplete the dissolved oxygen and put marine and plant life risk.
A measure of organic water quality contamination is its Biological Oxygen Demand or BOD5 . This is a measure of the amount of oxygen wastewater consumes over 5 days, at 20oC during the biological break down of organic contaminants, including FOG. The higher the BOD5 level, the more organic contaminants are present in the water
Most municipalities have set limits of 100-150 ppm for FOG (from vegetable or animal sources), 350 ppm for TSS and 300 ppm for BOD5. Commercial laundries which exceed these limits can have surcharges applied to their water bill; in proportional to the limits exceeded. For laundries using large volumes of water, this surcharge can be significant, ranging form hundreds to thousands of dollars per month, and so there is a strong economical motivation for abiding by the municipal by-laws.

Why are high levels of TSS undesirable?
Undissolved or insoluble matter, floating or suspended in water, imparts a cloudy appearance (turbidity) to it, and is referred to as total suspended solids or TSS.

Aside from its unpleasing esthetic appearance, the main concern with high TSS levels is in its ability to harbor harmful bacteria, such as coliforms. These microorganisms are harmful and can be fatal when ingested. They can readily attach themselves and hide on suspended solids, and are not readily disinfected.

How is Water pretreated before being discharged?
The topic of wastewater treatment is beyond the scope of this article. Future articles will address other technologies, including Reverse Osmosis, Ultrafiltration and Nanofiltration. These are technologies which depend on the size of the pores in semi-permeable membranes to filter unwanted impurities out of wastewater, and is carried out under high pressure. A current and very effective new technology for pre-treating laundry wastewater is through the use of a Dissolved Air Floatation Unit (DAF Unit).

How and Why Does a DAF Unit Work?
Recall that a large component of the BOD5 in laundry wastewater is due to the presence of fats, oils and greases. Reducing the FOG also lowers the BOD5 level. The economic benefit of using a DAF Unit is in avoiding a double sewer surcharge; one for excess BOD5 and the other for a FOG violation
FOG, being lighter than water has a natural tendency to float to the surface of a wastewater holding tank, from where it can be skimmed off. Since individual FOG droplets are very small and dispersed throughout a large tank of water, a considerable amount of time would be required for most of them to reach and concentrate at the surface. To resolve this problem, the heart of the DAF Unit consists of a compressed air-in-water tank which works as follows:.
Air is sparingly soluble in water under normal atmospheric pressure. However compressing air under high pressure, in a water-containing cylinder, results in a significant amount of the air being physically dissolved. This air enriched solution is then slowly released from the cylinder into the larger wastewater holding tank, and millions of tiny air bubbles come out of solution. A similar phenomena is seen when a soft drink bottle is slowly opened. Hundreds of gas bubbles are released which either adhere to the walls of the bottle or float to the surface.
As the tiny air bubbles rise to the surface, they attach themselves to the dispersed FOG particles, they lift them at an accelerated speed upwards towards the surface of the tank.
Total suspended solids (TSS), being heavier than water, would tend to sediment, within time, to the bottom of the holding tank. However, because many of the tiny air bubbles also surround and attach themselves to each suspended solid particle, they exert the same lifting effect, to the surface. In so doing, the turbidity of the solution is reduced, and so is the risk of paying a 3rd sewer surcharge for violation of TSS maximum levels.

Summary

  1. The major pollutants found in commercial laundry wastewater are FOG, TSS.
  2. High BOD5 are a reflection of high FOG, and both are regulated by municipal by-laws
  3. The most economical and effective way of removing FOG (and therefore reducing BOD5) is through the use of a DAF Unit.


 



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