Domestic Wastewater Problem

[luck3356]

I am trying to provide a solution for a domestic wastewater treatment problem that has unique constituents that are very difficult to get rid of. I believe that I can handle all the organics including TSS, BOD and nitrate levels with an Upflow Sludge Blanket Filtration (USBF) system, but I have no idea what to do with the other constituents that are present. Perhaps membrane filtration or reverse osmosis are options but I'm not even sure who to contact for that.

Please take a look at the following list of constituents and suggest a treatment solution. The first two numbers in the data set are successive readings the last number is the regulatory criteria.

Volatile Organics
Chloroform g/L 43.4 4.1 1.1
Dichlorobromomethane g/L 2.30 0.1 0.56
Inorganics
Aluminum g/L 104 162 87
Chromium (hexavalent) g/L NA 4 0.2
Copper g/L 9.5 10.1 3.8

Other Constituents
Chloride mg/L 111 32 106
Nitrate (as N) mg/L 37.0 26. 0 10
Specific Conductance (EC) uS/cm 1170 869 700
Total Dissolved Solids (TDS) mg/L 688 620 450

Chloride, electrical conductivity (EC), and total dissolved solids (TDS) are all indications of salt in the water. Hexavalent chromium is a carcinogen and for this reason is of concern because of the designated beneficial use of the receiving watershed as a domestic supply and, consequently, human health.

 

[cub3bead]

The volatile organics will air strip readily.

The metals will need to be dosed with sodium bisulfite (NaHSO3) to reduce the Cr+6 to Cr+3. Then you will need to precipitate it, usually done with caustic. this should get you Al, Cr and Cu below the reglatory limits. Note that the bisulfite and caustic will add TDS to the effluent.

RO and maybe NF will be needed to reduce the dissolved constituents to discharge limits. However, what are you going to do with the concentrate stream that will be 15 - 25% of the feed flow with a TSD that is 4 - 6.7 times the feed concentration?

 

[luck3356]

That is a very good question. I need to check the regulatory requirements but as long as we meet BOD, TSS and nitrate levels (10/10/10) we should be OK. What does your experience tell you about how the TDS will affect these levels?

 

[cub3bead]

RO rejects NO3 like any other salt so if your WWTP gives you 10 ppm in the effluent from the WWTP then your RO concetrate will have 40 - 67 ppm.

You will have to filter the TSS down much lower (<0.1 ppm)before the RO so the TSS in concentrate won't be a problem. TSS will foul the membrane surface and increase O&M costs dramatically.

You may only have to treat 50% of the flow with RO to meet your discharge limits.

Have you looked at a MBR yet. It combines a MF/UF with an activated sludge process. The MBR effluent can go straight to RO then.

 

[luck3356]

I have and here is the comparison I found:

USBF vs. MBR (Zenon)
• The USBF system has a smaller foot print than the Zenon MBR process and the capital investment is about
70% less than that of a Zenon MBR system.
• The Zenon process requires a biological treatment system and chemicals in order to remove carbonaceous
and nitrogenous oxygen demands in addition to the membranes used for TSS removal.
• MBR system requires a computerized control system that is essential for the operation of the system. Class
“A” experienced operators must operate and “fine tune” the MBR system twenty four hours per day seven
days per week.
• The USBF process is a self regulated system and very little, if any operator attention is required.
• The membranes in an MBR process must be cleaned on a daily basis by the use of “back-pulsing”. This is done
to reduce the possibility of fouling and debris collection on the membranes. The USBF process does not
require the additional controls or daily cleaning of the internal components.
• MBR system has a potential for fouling of the membranes by biological, chemical (sulfates, carbonates, etc.) or
physical contamination (hair, plastics, paper, etc.) associated with the waste stream.
• MBR system requires a fine mechanical bar screen (~1 mm) upstream of the unit to minimize the potential for
physical fouling of the membranes. The USBF uses a standard mechanical bar screen.
• The membranes in the MBR must be cleaned by the use of a chemical cleaning process on a monthly or
quarterly basis. The cleaning is done with NaOCl and acidic solutions, both of which must be handled and
used properly to prevent injury to the operators.
• The USBF process is simpler and requires less equipment, and electricity to operate. The USBF flows via
hydraulic gradeline (gravity) and the aeration is provided by fewer blowers. The MBR system on the other
hand requires permeate suction pumps and internal recycle pumps in addition to the blower requirements in
order to operate.
• MBR system typically requires the addition of chlorine in order to control filamentous growth within the
system, as opposed to control of the filamentous sludge by the process itself as is with the USBF process.
• The USBF process has an extended sludge age of 25 to 30 days with low microbial loading which produces less
excess, aerobically stabilized sludge and improves sludge structure and mechanical dewatering characteristics.
In summary, we believe that the USBF is a superior process for this application due to the following:
• Overall simpler process to operate
• Requires less electrical power
• Does note require computerized controls for operation
• No chemicals required for operation
• Less mechanical equipment to maintain
• Produces less sludge
• Requires less land area
The evaluation was prepared Mr. David E. Mahler, PE, VP, and Mr. Scott Breitenstein, P.E. of the CPH Engineers Inc. Orlando, Florida office.
Tel: 407 425-0452.

What is your opinion of MBR and do you have any experience with USBF?

 

[moltenmetal]

The numbers you have given for the VOCs are in micrograms per litre, not grams per litre as shows up on my screen, correct? Certainly hope so!

The chloroform and dichlorobromomethane are THMs, which are chlorination byproducts. The levels you're talking about are, or were, found in drinking water samples across a vast swath of North American locations which derived their drinking water from surface sources (i.e. rivers and lakes). Tough to imagine that a regulator is going to want you to treat the effluent water from a WWTP to cleaner standards than the source drinking water!

No matter- if you're doing aerobic bio, they're long gone because of air stripping.

 

[luck3356]

Micrograms per litre is correct. My mistake.

The regulators are trying to adhere to a future California Toxic Rule standard that will be enforced as of 2010. I am of the opinion that the State Water Quality Control Board will see eventually see the error of their ways and relax the standard. As it stands, I have been charged with providing a solution to this problem. This project is very political and I have made the exact point that you have to no avail.

 

[bimr]

cub3bead has provided some good recommendations.

Note that despite all of the benefits that you have proposed, the USBF process will never meet the discharge limit for TDS in your application because the USBF does not reduce the TDS.

Note sure why you seem to be enamored with the USBF process. Further, comparing the USBF process to a MBR process is silly since they are usually applied for different applications.

The MBR process basically replaces the clarifier on the back end of an activated sludge plant with a membrane separation process. So your comparison of USBF vs. MBR simplifies to a comparison of a clarifier vs. membrane filter.

If you have to desalt using an RO process, your best option is to use the MBR process that will also provide pretreatment for the RO.

If you have to reduce the TDS and you have no wastewater discharge, you will have to think about using an evaporator to reduce the volume of the RO reject stream.