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Industrial effluent treatment hydraulics
CHAPTER
Industrial effluent treatment hydraulics
20
CHAPTER OUTLINE Introduction ............................................................................................................249 Hydraulics and Layout .............................................................................................249 Hydraulic Design ....................................................................................................250 Flow Balancing .......................................................................................................250 Sludge Handling .....................................................................................................251
INTRODUCTION In my experience, the only difference between the hydraulics of municipal and industrial effluent treatment plants is that industrial effluent treatment plants are so often designed as afterthoughts, fed in short sharp batches by the main process into which most of the design effort went. In this chapter, I have attempted to offer some basic guidance on how to approach a design, for those conscientious designers who consider all aspects of the plant during the design process. The designer should always remember that effluent treatment plants (ETPs) are commonly seen as at best a nuisance by those who operate the main process. The designer should assume that any chemical used on the site will find its way to the ETP, and all means of transferring flow to the ETP will be run at their maximum capacity. Although businesses which supply industrial effluent treatment plant have some of the highest profit margins of anyone in the water industry, many such plants which I encounter are cheap and poorly constructed, based on best outdated technology, and designed with little ingenuity. In my experience the hydraulic aspect of these designs tends to be the weakest area of all.
HYDRAULICS AND LAYOUT Ideally the plant should be located at a low point on the site, to allow gravity feed from the main process. Channels may be favored for effluent collection, but
An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00020-5 © 2018 Elsevier Inc. All rights reserved.
249
250
CHAPTER 20 Industrial effluent treatment hydraulics
care should be taken in a situation where heavier-than-air flammable vapors might collect in such a channel, allowing the possibility of rapid spread of fire around a site. In my experience a very common approach is to feed the plant from several large pits or sumps around the site, emptied over a short period of time by large pumps. However, this approach is never optimal. The requirements of operation and commissioning for the ability to empty a tank quickly may make a pit a good idea, allowing a hydraulic buffer between the tank and the ETP, but the point of such a tank is not that it be emptied quickly. The pumps and pipework used to empty the pit would ideally be sized to restrict flow out of the pit. Experience suggests that they tend to be run at their maximum rated capacity, to maximize flexibility for the main process, or just to allow operators to attend to something else.
HYDRAULIC DESIGN Although, in the preceding section, I recommended deliberately restricting flow from catch pits into the plant, I would nonetheless recommend that all flow restrictions should be deliberate. The strong tendency to design industrial effluent treatment plants to be as cheap as possible should be resisted. Any successful main plant will tend, over time, to produce more effluent. An allowance for this should be built into the hydraulic design. The normal approaches to hydraulic design used by water treatment plant design specialists should be followed. A common mistake made by designers from other sectors trying their hand at effluent treatment plant design is to use active flow control, such as modulating valves, where specialists would use passive flow splitting chambers, and to use pipes where specialists would use channels. It is always better to hire a genuine specialist, though there are many more amateurs operating in industrial effluent treatment than there are in the municipal market. The relatively small size of the projects, and their one-off nature makes them unattractive to the larger established water treatment contractors.
FLOW BALANCING An industrial effluent treatment plant should normally be designed to handle the highest average daily flows experienced over the course of a year. The peak design throughput of the plant should be the average hourly flow on this peak day. A 24-h HRT in-line flow balancing tank upstream of the plant will allow accommodation of any instantaneous flows which are higher than the average.
Sludge Handling
The alternative (sizing the plant to handle instantaneous peak flow) will be likely to cost far more money, and be less controllable and flexible, due to turndown ratio restrictions and feed flow and composition variability.
SLUDGE HANDLING The particles in industrial effluent may be far denser than those found in municipal water and wastewater treatment plants. Industrial effluents can consequently produce very dense, hard to pump sludges. The high degree of variability of many industrial effluents can also mean that the density, viscosity, temperature, and chemical composition of sludges can vary dramatically over time. This needs to be considered when designing hoppers, tanks, and other sludge holding facilities, along with the type and ratings of pumps, pipes, valves, and so on. Rodding points or similar provision for solids settlement are recommended wherever wastewater or sludges are to be held and at bends in sludge pipework. Pipe runs between rodding points should be restricted to a length which can be rodded-through effectively.
251
Industrial effluent treatment hydraulics
20
CHAPTER OUTLINE Introduction ............................................................................................................249 Hydraulics and Layout .............................................................................................249 Hydraulic Design ....................................................................................................250 Flow Balancing .......................................................................................................250 Sludge Handling .....................................................................................................251
INTRODUCTION In my experience, the only difference between the hydraulics of municipal and industrial effluent treatment plants is that industrial effluent treatment plants are so often designed as afterthoughts, fed in short sharp batches by the main process into which most of the design effort went. In this chapter, I have attempted to offer some basic guidance on how to approach a design, for those conscientious designers who consider all aspects of the plant during the design process. The designer should always remember that effluent treatment plants (ETPs) are commonly seen as at best a nuisance by those who operate the main process. The designer should assume that any chemical used on the site will find its way to the ETP, and all means of transferring flow to the ETP will be run at their maximum capacity. Although businesses which supply industrial effluent treatment plant have some of the highest profit margins of anyone in the water industry, many such plants which I encounter are cheap and poorly constructed, based on best outdated technology, and designed with little ingenuity. In my experience the hydraulic aspect of these designs tends to be the weakest area of all.
HYDRAULICS AND LAYOUT Ideally the plant should be located at a low point on the site, to allow gravity feed from the main process. Channels may be favored for effluent collection, but
An Applied Guide to Water and Effluent Treatment Plant Design. DOI: https://doi.org/10.1016/B978-0-12-811309-7.00020-5 © 2018 Elsevier Inc. All rights reserved.
249
250
CHAPTER 20 Industrial effluent treatment hydraulics
care should be taken in a situation where heavier-than-air flammable vapors might collect in such a channel, allowing the possibility of rapid spread of fire around a site. In my experience a very common approach is to feed the plant from several large pits or sumps around the site, emptied over a short period of time by large pumps. However, this approach is never optimal. The requirements of operation and commissioning for the ability to empty a tank quickly may make a pit a good idea, allowing a hydraulic buffer between the tank and the ETP, but the point of such a tank is not that it be emptied quickly. The pumps and pipework used to empty the pit would ideally be sized to restrict flow out of the pit. Experience suggests that they tend to be run at their maximum rated capacity, to maximize flexibility for the main process, or just to allow operators to attend to something else.
HYDRAULIC DESIGN Although, in the preceding section, I recommended deliberately restricting flow from catch pits into the plant, I would nonetheless recommend that all flow restrictions should be deliberate. The strong tendency to design industrial effluent treatment plants to be as cheap as possible should be resisted. Any successful main plant will tend, over time, to produce more effluent. An allowance for this should be built into the hydraulic design. The normal approaches to hydraulic design used by water treatment plant design specialists should be followed. A common mistake made by designers from other sectors trying their hand at effluent treatment plant design is to use active flow control, such as modulating valves, where specialists would use passive flow splitting chambers, and to use pipes where specialists would use channels. It is always better to hire a genuine specialist, though there are many more amateurs operating in industrial effluent treatment than there are in the municipal market. The relatively small size of the projects, and their one-off nature makes them unattractive to the larger established water treatment contractors.
FLOW BALANCING An industrial effluent treatment plant should normally be designed to handle the highest average daily flows experienced over the course of a year. The peak design throughput of the plant should be the average hourly flow on this peak day. A 24-h HRT in-line flow balancing tank upstream of the plant will allow accommodation of any instantaneous flows which are higher than the average.
Sludge Handling
The alternative (sizing the plant to handle instantaneous peak flow) will be likely to cost far more money, and be less controllable and flexible, due to turndown ratio restrictions and feed flow and composition variability.
SLUDGE HANDLING The particles in industrial effluent may be far denser than those found in municipal water and wastewater treatment plants. Industrial effluents can consequently produce very dense, hard to pump sludges. The high degree of variability of many industrial effluents can also mean that the density, viscosity, temperature, and chemical composition of sludges can vary dramatically over time. This needs to be considered when designing hoppers, tanks, and other sludge holding facilities, along with the type and ratings of pumps, pipes, valves, and so on. Rodding points or similar provision for solids settlement are recommended wherever wastewater or sludges are to be held and at bends in sludge pipework. Pipe runs between rodding points should be restricted to a length which can be rodded-through effectively.
251