Special on : TOC

( article : TOC-E-5 )

Managing waste water pollution effectively

Analysis of the systems used to monitor and control organic loadings in waste water, storm water and condensates.

Shay Hancock

Introduction
The use of Total Organic Carbon monitors in the effective monitoring of waste water storm or condensates and as means of detecting spillages and organic load has increased significantly in recent times.
Though the essential value of COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand) measurements in the characterisation of industrial effluents and the control of treatment plants is undeniable it has become more common place in recent times to express organic pollution in terms of TOC Total Organic Carbon measurements.

TOC Analysis
The use of TOC as an effective means of monitoring offers several advantages; faster analysis, continuously available results enabling diversionary tactics to be deployed in good time to avoid discharge of high organics.
The net value is faster results, a higher level of automation and reliability and more effective control.
Many papers have been previously written that outline the correlation of COD/BOD v's TOC.
The conclusions that may be drawn clearly indicate that the control of organic loading in waste water and industrial wastes can be suitably conducted with the use of an effective TOC monitor.
By utilising the TOC value and an experimentally found correlation factor the COD value can be expressed to good approximation.
The COD/TOC factor varies for each type of effluent : for example in a given stream the ratio will be higher in raw water than in biologically purified water. This is likely to be due to the decreased dichromate oxidability of the organic residues after biological oxidation.

The design of TOC monitors as they have evolved over the last 30 years vary significantly in terms of the techniques employed and the type of detection systems they incorporate.
For the purpose of this article however they may be broadly categorised as follows, high temperature combustion systems and elevated temperature UV promoted chemical oxidation systems.
Whereas high temperature combustion systems have found good application in laboratory TOC analysis or in some cases in on line analysis of clean waters, the UV systems are more universally suited to all applications.

UV-Promoted Chemical Oxidation
For on-line analysis of waste water containing salts or corrosive products the elevated temperature UV promoted chemical oxidation system has proven to be more successful and offers a higher level of reliability.
The UV promoted chemical oxidation system overcomes problems frequently associated with corrosion, catalyst poisoning and batchwise sampling.
The UV chemical oxidation systems have been more broadly adopted by the chemical, pharmaceutical and dairy industry in Ireland.Many other applications exist besides wastewater for TOC measurements and control.

One such application is cooling water system monitoring.
This includes cooling water tower recycle, heat excharger leak detection/protection, cooling water from rivers lakes or sea, and of course condensate monitoring.
Many closed cooling water systems are susceptible to leakage from various pieces of process equipment.
The material lost into the system represents lost revenue in four different ways.
Lost product yield, the cost of treatment or removal, or blowdown water replacement.
In air cooling towers loss of hydrocarbons into the atmosphere. Loss of cooling efficiency due to heat excharger fouling.
On the other hand in open systems whose supply and return is a natural water body the additional concern is that of major spillages due to catastrophic heat exchange failure.
The solution is to monitor the cooling water with a fast and reliable device at the inlet for TC (TOC) as an indication of the system leak or spill.
TC is preferential because of the varying amounts of carbonates especially in natural waters.
On open systems a differential reading is advisable for this very reason as well as the fact that other parties may have already spilled into the same body of water.
In addition the analyser signal may be used to divert any contaminated water, generated as a result of spillage, to a holding pond of lagoon for subsequent treatment and discharge.
Monitoring immediately downstream of critical pieces of equipment in order to detect even the most minute leak allows convenient scheduling of maintenance.
The TOC monitor has been found a very valuable application in the monitoring of steam condensate return purity.
Collecting and recycling steam condensate is desirable and saves energy and money.
However leaks may develop that will deposit hydrocarbons, etc., into the condensate.
Should some of these soluble and partially soluble contaminants find their way back to the boiler without being detected and removed they will propably hasten the demise of the boiler.
These contaminats will foul the inner surfaces of the fired tubes thus lowering their heat exchange efficiency and block off passages.
In order to overcome the lower heat transfer rate, higher fire box temperatures are necessary, thereby hastening tube burn out.
Once again an effective solution is offered by monitoring the condensate for either TC (Total Carbon) or TOC (Total Organic Carbon).
The TC is preferred because of the possible build up of inorganic carbonates which can cause problems.
The typical range for this measurement is 0 - 10 ppm TC.
On a high TC/TOC, perhaps from 5 to 10 ppm, condensate should be diverted and an alarm system should warn the operators that there is a leak or some other malfunction of the condensate system.
These are just some examples of the type of applications available to an effective on line TC/TOC monitor.
The many advantages are clear. They are defined in terms of monitoring for compliance to legislation and also for effective waste management and control.

 

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