Blog Archives

"Paper Mill with lagoon system facing pressure to install activated sludge" - Should the change ~ or can existing system be developed for more efficiency?

8/31/2014

Pulp & Paper Lagoon System

A large paper mill in southern Georgia is facing increasing pressure to reduce color and odors of their effluent entering into a river system where their daily discharge makes a significant change in river aesthetic quality below the mill. The immediate suggestion by experts if for the mill to replace their large aerated lagoon system with the "latest" in activated sludge treatment systems. Such a system would be smaller in foot print - freeing up land, but would cost hundreds of millions to build and have very high ongoing operational costs. My questions is ~ can this paper mill use existing lagoons more efficiently and improve effluent quality with less initial capital outlay and no significant increase in operational cots?

Most paper mill lagoons in the US contain earthen basins for water retention and high speed surface aerators to providing for oxygen used by the microbes. Biological solids are removed in polishing ponds which require periodic dredging of sludge to maintain volume.

While the high speed aerators are efficient in adding dissolved oxygen - approximately 2 lb O2/hp/hr. They are less efficient in mixing and result in sludge buildup just outside the zone of influence. As sludge builds, wastewater treatment volume is reduced and short-circuiting of flows becomes an issue. In ares with electric lines and aerators, the mill cannot easily dredge the solids so occasional aerators are moved to help prevent excessive sludge buildup.

While I am not involved with the mill's upgrade proces, I would usually start my optimization process with the following steps.

Clarifier Bed Depth - What should I target?

8/26/2014

Secondary clarifiers are important for two reasons: (1) remove biological solids (TSS) from effluent discharge & (2) allow for recycle of biosolids to the head of the biological treatment unit thereby maintaining a higher population of microbes than seen in lagoon systems.

Proper secondary clarifier operation is key to successful biological treatment in activated sludge. Normally we design clarifiers for a hydraulic residence time of 2 - 6 hours all based on actual or anticipated solids flux testing. If residence time is too low, we see solids carryover the weir and high TSS in the effluent. Additionally, the return sludge (RAS) concentration is low and beneficial biosolids are being lost to the effluent via washout. If hydraulic loadings are creating too low a retention time, modifications to the clarifier can reduce water velocity allowing for improved function. Other options include adding polymers to improve floc density which gives more rapid settling. Akin to adding polymers, certain bioaugmentation cultures contain polymer producing microbes that grow in the system increasing floc density which can reduce overall operational costs - but these cultures must fit the system and be evaluated much like polymer jar testing.

In some systems we see the retention time in the clarifiers is too long, resulting in floating sludge (denitrification) and excessive torque on the rake caused by dense biological solids on the bottom. To ensure this does not happen, increase the recycling rate and maintain a bed depth below 2 feet in most clarifier designs.

Again - all operations are site specific depending upon influent flows, design parameters, and current system biological/chemical conditions. Evaluation of settling rates (SVI, solids flux), effluent turbidity/TSS, clarifier bed depth, RAS & MLSS Concentrations, and flow rates can allow for modeling of what happens with variation in influent flows and concentrations and can improve daily operations while reducing treatment costs.

Aquaculture can improve stream water quality!

8/18/2014

Oyster Beds

For years, studies have been done to show the negative impacts of aquaculture on receiving streams. Environmental damages include destruction of wetlands, increased nutrients (nitrogen & phosphorous) in the effluent, and organic pollution from animal wastes.

A recent study of the Potamac River by researchers from the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS) used modeling and scientific evaluations to estimate that virtually all nitrogen pollution (leading cause of algae blooms in brackish water such as the Chesapeake Bay) can be eliminated if 40% of the river bed were used for shellfish cultivation. Even using just 15- 25% of the river bed can remove up to 50% of the nutrients.

The nutrients are removed by the filter feeding process of the oysters. The removal of organic particles and nutrients will result in lower nutrient loadings entering the imperiled areas of the Chesapeake Bay. This filtering would also increase dissolved oxygen residuals, reducing the dead zones often seen in polluted waters.

Selecting proper system for biological treatment

8/15/2014

On a recent discussion group, someone asked about biological treatment of starch contaminated wastes. Instantly the vendors lined up to sell their technologies including additives, equipment, and various treatment systems. Only in a few cases people actually asked key questions that need to be considered before selecting a treatment system. Always start with the following questions:

Organic make up of waste: In this case it was starch

Concentration (BOD/COD) If COD >1500 mg/L - investigate anaerobic treatment, with COD < 1300 mg/L often aerobic is easier to stabilize and maintain.
Other major influent parameters - pH, temp, nitrogen, phosphorus, alkalinity
Are their any inhibitory, resistant, or toxic compounds?
Treatment objectives & permit limits

Only once you have a full characterization of the influent wastes, can you begin to select the proper biological treatment process. The key point being - you fit the treatment system to the waste stream and treatment objectives.

Settling Tests (SV30) - Procedure & Interpreting Results

8/12/2014

Impact of Cylinders on SV30

In activated sludge wastewater treatment one of the most commonly run and important tests is the settleometer or sludge settling test. While there are special 2L plastic cylinders made for this test, most facilities use a 1L graduated cylinder for settling tests. No matter which cylinder you use, make sure you use the same cylinder to ensure consistency and useful results.

Procedure
Add 1000 ml of mixed liquor to the cylinder. Start timer an allow the product to settle. You can take readings every 5 minutes, but often we just take a reading after 30 minutes. By tanking readings every 5 minutes, you get a settling curve that provides more information on how the floc performs in secondary clarification. While we normally use the SV30 number, in systems with changing MLSS concentration or comparing different systems, we would do the SVI calculation. SVI > 150 is con

SVI = SV30/ MLSS (in grams)

Interpreting Results

If the floc settles rapidly but leaves pin floc in solution or fine floc floating on the top, it indicates an "old sludge". In the secondary clarifier this results in undesirable turbidity and TSS passing into the effluent. As a result, you should increase wasting rates.
If the floc does not compact as much as normal, you may have young sludge, filamentous or non-filamentous bulking. Young sludge will also have a high oxygen uptake rate and many bacteria cells in free-solution outside the floc. In this case, it will take time for the biomass to develop and can indicate increased loadings. If the increased loadings are causing problems with solids carryover in the clarifier, instead of using secondary polymers or restricting flow it could be time to add bioaugmentation products to shorten the time for floc development in the biological treatment unit.
Filaments and non-filaments are a different situation than young sludge. Each filament and type of bulking is caused by a number of factors. If the microscopic exam shows substantial filaments and or diffuse floc, it is best to look at the biological unit for the causative factor and make changes to promote better floc formation thereby reducing growth pressures for filaments.

Spirochetes - what are they and what do they indicate in my microscopic exam

8/6/2014

Image from www.vanleeuwenhoek.com

Spirillium or spirochetes are spiral shaped microorganisms that move with a unique cork-screw movement through the sample. Their rapid movement makes then easily observed in water samples.

You often see spirochetes when there are "septic" conditions.

Septicity is produced by anoxic or anaerobic conditions allowing for fermentative respiration. During fermentative resipiration, bacteria produce organic acids as the end product of metabolism.

Spirochetes may also be common with influents containing organic acids (common in brewing, fruit and vegetable processing, winery, etc).

No matter what the source, high levels of organic acids can cause problems with filaments that thrive on the low dissolved oxygen and readily soluble substrate. This is why systems with high influent organic acid loadings often see fewer filament problems with a complete mix reactor or step-feed in plug flow reactors. Another source of organic acids is a long residence time in the primary clarifier or upstream lift stations.