This Month’s Puzzler
We’re concerned about the water supply at our small juice bottling operation. We rely on a local river for water, just like our city does; we consume a lot of water, 8,000 gpm. Originally, we used well water but were forced to tap into the river to meet our demand. Now, because of a recent drought, we’re being asked to cut back.
In addition, the city is unhappy with our wastewater — both the volume we send to municipal treatment and its tinted color — and would like us to decrease the output.
Corporate management is concerned about the cost of on-site water treatment. We’re seeing trace heavy metals like lead and cadmium and blips of high biomass in the fresh water and, sometimes, in the trout and salmon found in the river.
What ideas can you suggest for reducing our water consumption and wastewater volume and for treating the wastewater? Evaluate Your Process
A common but an important challenge faced by many industries is to reduce the water footprint of their existing facilities. To begin with this initiative, first and foremost you need to study the process flow and mark up areas using water. Then, analyze the process flow carefully to devise new flow interactions that can help decrease the water usage. Many times, low water quality results in a greater usage of water. Finding another source of water, possibly from within the process, and treating it before it’s mixed with the product is one of the options to reduce water consumption.
There exist multiple wastewater methods, such as bio-membrane filtration and packed-bed bio scrubbers, to treat the heavy metals. Research in the area of wastewater treatment has led to the discovery of many different kinds of packed bed that give a high removal efficiency in terms of contaminants.
Munira Marvi, engineer
MACH Engineering, Houston
Look For Low-Hanging Fruit
Try this approach:
1. Take a walk around the plant to identify areas where water is wasted (leaks, open hoses, in-plant water meters not working, etc.). This point is obvious but, ironically, easily missed.
2. Review the process flow diagram to determine major water users. If water serves as a cooling medium, evaluate the feasibility of using air cooling.
3. If discoloration of treated wastewater is a recent problem, check operating data to see if any changes in operations caused discoloration.
4. Review your steam balance. Frequent or prolonged venting of steam would indicate that it’s out-of-balance and there’s an opportunity to reduce water use.
5. Assess cleaning operations, such as cleaning and washing bottles. Critically look at each step and evaluate the possibility of reducing water use while maintaining an acceptable level of purity of washed bottles.
GC Shah, senior advisor
Wood Group, Mustang, Inc., Houston
Use Gray Water
It sounds like you’re being squeezed between the city and corporate management. Corporate probably doesn’t see the whole picture. Your real problem is the city: if they cut off your water, you’re out of business.
The best solution is to start considering water the same way you do the ammonia used in a refrigeration system — i.e., as a costly resource. You can’t do anything about your product; you need water to make juice. Take a look at what’s possible to reduce your pulp recovery step’s contribution to wastewater; perhaps you even can reclaim this water.
Pipelines aren’t the answer because they become uneconomical at about 35 miles from a water source. For instance, Chicago supplies outer suburbs with water via a pipeline and the prices have gone up significantly; some suburbs have abandoned plans to extend pipelines as uneconomical.
Then, there’s the politics of managing corporate image. To address corporate’s needs, look into “gray water,” i.e., recycle. For example, in many plants the seal water for pumps often goes straight to a sewer. These streams typically are small but not insignificant. One plant I know consumed over 200 gpm — with the plant shutdown! Using a rule-of-thumb estimate based on pump nominal flow rate, Q, and ultimately shaft size, seal water flow is:
Q < 50 gpm — 0.4 gpm;
50 < Q < 100 gpm — 0.7 gpm;
100 < Q < 350 gpm — 1 gpm;
350 < Q < 600 gpm — 1.6 gpm; and
600 < Q < 1,500 gpm — 2.1 gpm.
Other sources of grey water include cooling tower sump water, scrubber water, washdown water (usually food plants generate a considerable quantity) and waste sanitary water from toilets, showers and sinks.
Now, consider wastewater treatment. While biological bed treatment is tempting, bugs don’t efficiently digest lead and other heavy metals without producing something worse. Reverse osmosis (RO) and other treatment methods such as activated carbon (AC) have shown promise. If the incoming water is mineral-rich, i.e., with a high level of total dissolved solids (TDS), RO and AC will be expensive options; well water tends to have higher TDS than surface waters like those from rivers and lakes. Chlorine and peroxide treatment can eliminate a high biomass level before TDS removal. Papermills have shown that tinting can be removed by AC — but take care to avoid particulates. As for the trout, I suggest marinating in brandy, olive oil, ginger and soy sauce before serving.
One idea that meets both goals of reducing city water demand and improving corporate image might be to help the city cut water usage city-wide and even in the farms in the area. It’s certainly worth a try.
Dirk Willard, consultant
One thing to suggest is to have good effluent water analysis with respect to cations, anions, COD, TSS and pH. Based on this analysis, one can look into ways to reuse the effluent for many non-potable uses. This problem is common in many processes where water costs and limitations is cause for concern and working with water company with common goals and expertise can certainly help. There are many ways of reducing the water from bottle washing and potentially reusing based on water chemistry.
Jitendra (Jet) Shah, Ecolab Fellow
Cooling problems plague our chemical plant, especially on days when the temperature rises above 95°F and the humidity exceeds 85% (see Figure 1).
Corporate engineering completed a plant expansion a couple of winters ago; since then, cooling has become a recurring problem every summer starting in June and going on until late September.
In response, at the suggestion of corporate, we put in larger batch reactors and distillation towers and added a cooling tower to supplement the old 1970s one that has served us so well.
Figure 1. The temperature differential of the feedstock and of ammonia in the refrigeration system both have dropped.
It’s probably worth mentioning that we changed our feedstock source. We had been making it ourselves but several near-misses at the feedstock unit prompted corporate to close it down. Now, we import feedstock from offshore; we’re still waiting to establish quality control procedures on the new feedstock.
I’m concerned about the low temperature differential across the ammonia refrigeration system: the vaporizer (NH3feedstock) and condenser (NH3/tower water) don’t have the same differential temperature as before. Before the expansion, we saw a 17°F change in feedstock temperature, now it’s only 10°F. The ammonia differential had been 12°F but currently is 7°F. Also, I noticed that the ammonia compressor is running hotter and the inlet temperature to the ammonia condenser also is a little hotter. What does all this mean? What can we do?
Send us your comments, suggestions or solutions for this question by October 12, 2018. We’ll include as many of them as possible in the November 2018 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.