Lesson 30: Biosolids Stabilization/Digestion Calculations

Lesson 30:

Biosolids Stabilization/Digestion Calculations

Objective

In this lesson we will learn the following calculations:

Solids loading
Digestion and Retention times
Organic loading
pH adjustment
Oxygen Uptake Rate
Seed volume
Volatile matter reduction (%)

Lecture

Biosolids Stabilization

A major problem for wastewater treatment plants is the disposal of biosolids into the environment without causing damage or a nuisance. Untreated biosolids are more difficult to dispose of. Untreated raw biosolids must be stabilized to minimize disposal problems. The stabilization of organic matter is accomlished biologically using a variety of organisms. These microorganisms convert the colloidal and dissolved organic matter into various gases and protoplasm. Since protoplasm has a specific gravity slightly higher than water, it can be removed from the treated liquid by gravity.

Biosolids digestion is a process in which biochemical decomposition of the organic solids occurs. In the decomposition process, the organics are converted into simpler and more stable substances. Digestion also reduces the total mass or weight of biosolids, destroys pathogens, and makes it easier to dry or dewater the biosolids. Well-digested biosolids have the appearance and characteristics of a rich-potting soil. Biosolids may be digested under aerobic or anaerobic conditions. Most large municipal wastewater treatment plants use anaerobic digestion, whereas aerobic digestion finds application usually in small, package-activated biosolids treatment systems. In the anaerobic process, biosolids enter the sealed digester, where organic matter decomposes anaerobically.

Once the solids have been thickened they are ready to be stabilized. At this point, the solids have only been thickened and they are the waste products of the liquid portion of the treatment process. There is a large amount of volatile organic matter that needs to be stabilized. By stabilizing the biosolids it will help reduce odors and destroy pathogens.

Aerobic Digestion

Aerobic digestion is very similar to the aeration tanks used in activated sludge systems. The primary and/or secondary sludge is digested aerobically, meaning that aerobic bacteria will break down the organic matter. The digesters can either be rectangular or round. Since the bacteria are aerobic air must be applied to the digester to supply oxygen. One major difference between the aeration tank in the activated sludge system and the aerobic digester is that there is not a continuous supply of fresh BOD. In the digester, there is no fresh wastewater coming in, only the settled solids. In the digester, the aerobic bacteria will be able to breathe, but with no food source. This means they will undergo endogenous respiration, which is a situation in which living organisms oxidize some of their own cellular mass instead of new organic matter. By breaking down their own cell mass, this reduces the amount of volatile suspended solids. Aerobic digesters will have a longer detention time, typically 30 days or more, and common volatile solids can be reduced around 45-70%.

Chemical stabilization is achieved by adding calcium hydroxide (Ca(OH)₂). It is commonly known as slaked lime. Adding the lime will raise the pH of the sludge to the point where biological activity is drastically reduced. This is much different from digestion because the organic matter is not reduced. The lime temporarily halts the biological activity and thus stabilizes the sludge. Chemical stabilization is not as common due to the high costs, regulations and environmental impacts of handling chemicals.

Anaerobic Digestion

The purpose of anaerobic digestion is the same as aerobic digestion: to stabilize the organic matter, reduce the volume and to eliminate pathogenic organisms. Remember that anaerobic means the environment has no free or combined sources of oxygen, so bacteria must find a different source of respiration. Anaerobic digestion is a two-step biological treatment process. The first step is done by a group of bacteria that breakdown solids to form volatile acids. The second step is another group of bacteria breaking down those volatile acids to form methane, carbon dioxide and water. When checking the operation of a digester, pH is a critical component. The first step of the digestion process is to create volatile acids. Excessive acids will cause a drop in pH. The acceptable pH range is between 6.6 and 7.6. If the pH drops below this, that is a sign that there are not enough methane forming bacteria to break down the volatile acids.

There are three different types of anerobic digesters, based on the operating temperature:

Psychrophilic: operates between 50 - 68°F
Mesophilic: operates between 85 - 100°F
Thermophilic: operates between 120 - 135°F

Equipment used in anaerobic digestion includes an anaerobic digester that is either floating or has a fixed cover. These include biosolids pumps for addition and withdrawal of biosolids, as well as heating equipment such as heat exchangers, heaters and pumps, and mixing equipment for recirculation. Typical accessories includes gas storage, cleaning equipment, and safety equipment such as vacuum and pressure relief devices, flame traps, and explosion-proff electrical equipment. Remember the anaerobic process breaks down volatile acids to form methane, carbon dioxide and water, which is very flammable.

Process Control Calculations

Aerobic Digestion

Volatile Solids Loading

The purpose of aerobic digestion is to stabilize organic matter, to reduce volume, and to eliminate pathogenic organisms. Aerobic digestion is similar to the activated biosolids process. Biosolids are usually aerated for 20 days or more and the volatile solids are reduced through biological activity during this time. The volatile solids, or organic matter, loading for the aerobic digester is expressed in pounds of volatile solids entering the digester per day per cubic foot of digester capacity. The typical loading rate for volatile solids is 0.02 to 0.14 lb/day/ft³. This can be demonstrated with the following equation:

Example:

The aerobic digester is 25 ft in diameter and has an operating depth of 15 ft. The biosolids that are added to the digester daily contains 1,625 lb of volatile solids. What is the volatile solids loading in pounds per day per cubic foot (lb/day/ft³)?

First determine the volume of the aerobic digester:

Volume, ft³= 0.785 x (Diameter)² x Depth

Volume, ft³= 0.785 x (25 ft)² x 15 ft

Volume, ft³= 7,359.38 ft³

Now plug the values into the equation to determine the volatile solids loading for the aerobic digester:

As you can see the volatile solids loading rate is higher than the optimum range of 0.02 to 0.15 lb/day/ft³. This means you need more microbes to consume the organic material, or a larger aeration basin volume.

Digestion Time

The theoretical time the biosolids remain in the aerobic digester can be determined using the following formula for digestion time:

The typical range for biosolids to remain in aerobic digestion is 10 to 20 days.

Example:

The digester volume is 20,000 ft³. Biosolids are added to the digester at the rate of 12,800 gpd. What is the digestion time in days?

Since the equation needs the volume in gallons you must first convert the digester volume from cubic feet to gallons:

(20,000 ft³) x (7.48 gal/1 ft³) = 149,600 gal

Now plug the values into the formula for digestion time:

pH Adjustment

If you do need to adjust the pH of the aerobic digester the operator must perform a lab test to determine the amount of alkalinity required to raise the pH to the desired level. The results of the lab test must then be converted to the actual quantity required by the digester using the following formula:

The optimum pH range for the aerobic digester is between 7.0 and 7.4, as close to neutral as possible.

Example:

Contents of an aerobic digester were tested in the lab. If 255 mg of lime is shown to increase a 50 mL sample of the contents to pH 7.1, how many pounds of lime will be required to increase the digester pH to 7.3 if the digester volume is 320,000 gallons?

Since the volume of the sample is given in mL, you will need to convert it to liters before plugging the values into the formula:

(50 mL) x (1 L/1000 mL) = 0.05 L

Now plug the values into the equation to determine how many pounds of lime you will need to add to the digester:

You will need to add 13,604.99 pounds of lime to the digester to raise the pH to 7.3.

Oxygen Uptake Rate (OUR)

Biological treatment of wastewater in the aerated stabilization basin and in the activated sludge system, is based on the ability of microbes to utilize dissolved oxygen (DO) in the process of breaking down the soluble organic substances present in the process effluent. The oxygen uptake rate (OUR) test is used to measure the metabolic activity of organisms in the system. Microbes use oxygen as they consume food in an aerobic system. The rate at which they use oxygen is an indicator of the biological activity of the system. High OUR measurements indicate high biological activity, whereas low OUR rates indicate low biological activity. This analysis is based on a series of DO measurements taken over a period of time. By knowing the OUR value, you can determine how active your microbes are in the aeration tank and whether they are consuming the oxygen you provide for them to biodegrade the organic matter in the incoming influent. The oxygen uptake rate, or oxygen consumption rate, can be determined with the following equation:

The test is most valuable for plant operations when combined with volatile suspended solids data. Combining oxygen uptake and suspended solids data yields a value called the specific oxygen uptake rate (SOUR).

SOUR describes the amount of oxygen used by the microbes to consume one gram of food and is reported in mg/L of oxygen used per gram of organic material per hour. Toxic or high organic loads can often be detected before severe deterioration of effluent quality occurs. Changes in the SOUR on effluent samples will indicate changes in loading. Through experience, it has been determined that the optimum range of SOUR is usually within 8 - 20 mg/g/hr. The value is indicated and measured in terms of unit of mg of oxygen consumed per liter in an hour, based on mg of MLVSS. The specific oxygen uptake rate can be determined with the following formula:

If your SOUR is above the recommended optimal range, it basically means that your aeration system has too much food (BOD load) and too few microbes (represented by MLVSS) to consume it. This also gives you an indication that your sludge is too young and if not addressed, floating suspended solids that do not settle fast enough will cause carry over of the particles together with your discharge effluent. The opposite condition happens when SOUR drops below the 8 recommended limit, which means that there is now insufficient food to support the microbe's growth and then cause the sludge to settle too quickly and lead to pinpoint floc.

Example:

What is the SOUR in mg/g VSS/hr at a facility where the MLVSS was measured at 1820 mg/L? During the analysis, the initial DO was 8.8 mg/L and 15 minutes later the DO is 6.2 mg/L.

First, you must determine the amount of oxygen used during this five minute period. This is determined using the oxygen uptake rate equation:

You have been given the MLVSS, but it's in mg/L. The SOUR equation needs the measurement to be in g/L. Let's do that conversion:

(1820 mg/L) x (1 g/1000 mg) = 1.82 g/L

Now you can plug in the values to determine the SOUR of the sample:

As you can see, this plant is running within the optimal range of 8 - 10 mg/g/hr.

Anaerobic Digestion

Seed Volume

Sometimes you will need to add seed to the digester in order to achieve normal operation, especially if it is a new digester. If this is the case you can determine the required seed volume, in gallons, using the following equation:

Seed volume, gal = Digester volume, gal x % Seed

Example:

The new digester requires a 36% seed to achieve normal operation within the allotted time. If the digester volume is 45,000 ft³, how many gallons of seed material will be required?

Since the volume is given in cubic feet, you will need to convert it to gallons before plugging in the values:

(45,000 ft³) x (7.48 gal/1 ft³) = 336,600 gal

Now plug the values into the formula:

Seed volume, gal = Digester volume, gal x % Seed

Seed volume, gal = 336,600 gal x 0.36

Seed volume, gal = 121,176 gal

This means you will need to add 121,176 gallons of seed material to the new digester to acheive normal operation.

Organic Loading Rate

Given the volumes of feed sludge, sludge concentrations, volatile solids, and the volume of the digester, you can calculate the organic loading rate of the digester in pounds per day per cubic foot. The typical organic loading rate is 0.02 to 0.14 lb/VSS/day/ft³. This can be done using the following equation:

Example:

If a 50,000 ft³ digester receives 6200 gpd of raw sludge that has a concentration of 4% and a volatile solids concentration of 75%, what is the organic loading rate in lb/day/ft³?

First, determine the volatile solids loading in lb/day:

Votaile solids, lb/day = Raw sludge volume, gpd x Sludge conc., %, x 8.34 lb/gal x VS %

Volatile solids, lb/day = 6200 gpd x 0.04 x 8.34 lb/gal x 0.75

Volatile solids, lb/day = 1,551.24 lb/day

Now plug in the values to determine the organic loading rate on the digester:

This plant is operating within the optimal volatile solids organic loading rate of 0.02 to 0.14 lb/day/ft³.

Volatile Acids to Alkalinity Ratio

Remember that the pH of the digester is very important for normal operations to continue. Determining the ratio between the volatile acids content and the alkalinity of the digester will help determine if the digester is operating properly. Volatile acids are fatty acids (organic) that are soluble in water and are expressed as milligrams of equivalent acetic acid and indicates the health of the digester. In a normal or healthy digester, the volatile acids will be used as the food for the methane formers. The production of organic acids depends on the volume of solids fed to the digester. The typical range for volatile acids in a primary digester is between 50 and 300 mg/L. When concentrations climb avoe 300 mg/L, the digester could be overloaded or experiencing other problems.

Alkalinity is the buffering capacity of water to neutralize acids and is expressed in milligrams of equivalent calcium carbonate per liter. The methane formers in anaerobic digestion are affected by small pH changes, while the acid producers can function fine across a wide pH range. Digestion stability depends on the buffering capacity of the digester contents. Higher alkalinity values indicate a greater capacity for resisting pH changes. The typical alkalinity value can range between 1500 and 5000 mg/L.

When examined together, we can measure and control the digestion process. By using the volatile acids to alkalinity ratio you can see a snapshot of the digester operation. Maintaining a consistent ratio of less than 0.35 ensures that conditions are correct for proper digester operation. The ratio in a well-operated digester ranges between 0.1 and 0.35. If the ratio exceeds 0.35, it is an indication of issues such as increased organic loading, hydraulic overloading, etc.

This can be determined through the following equation:

Example:

If the digester contains 320 mg/L volatile acids and 1750 mg/L alkalinity, what is the volatile acids/alkalinity ratio for the digester?

Simply plug the values into the equation:

If this ratio increases past 0.35 in the digester it is normally an indication of potential changes in the operating condition of the digester.

Retention Time

The biosolids retention time is the length of time the biosolids remain in the anerobic digester. This is similiar to the digestion time covered under aerobic digestion. The retention time is dependent upon the temperature range (mesophilic or thermophilic). Mesophilic organisms grow best in the temperature range of 30 - 38°C (85 - 100°F). Most anaerobic digestion processes at treatment plants operate in the mesophilic range. It is important for operators to maintain the temperature within a narrow range, typically 35 - 37°C (95 - 98°F). The temperature must not fluctuate more than 0.6°C (1°F) per day. The solids retention time for mesophilic digesters range from 10 to 30 days.

Thermophilic digestion grow best in the temperature range of 50 - 60°C (122 - 140°F). Temperature variations are especially hard on thermophilic organisms. A digester system that operates at this higher temperature range requires a shorter solids retention time than the mesophilic digester, ranging between 5 and 12 days.

This time can be determined with the following formula:

Example:

Biosolids are added to a 450,000 gallon digester at the rate of 13,500 gpd. What is the biosolids retention time in this digester?

Since the volume is given in gallons and the volume added is in gallons per day, there is no need to do any conversions. Just plug in the values:

This tells you the digester is operating within the mesophilic temperature range, with has a typical retention time of 10 to 30 days. In this case, it's taking a little longer, so another digester may need to be added to compensate for the high flow of biosolids.

Volatile Matter Reduction (%)

The expected reduction of volatile solids in a properly operating digester is 40 to 60% of the total volatile solids present in the raw sludge feed. The volatile solids in the feed sludge would be around 70 to 75% while the digested sludge would be around 45 to 50% volatile solids.

Because of the changes occurring during biosolids digestion, the following formula is used to determine percent volatile matter reduction in the biosolids:

*The values must all be entered as a decimal.

Let's watch a video showing how to determine the volatile solids reduction.

Example:

Determine the percent volatile matter reduction for a digester when the raw biosolids volatile matter is 65% and digested biosolids volatile matter is 48%.

*Hint: The raw biosolids is the VS in, while the digested biosolids is VS out. Also remember, all percents are entered as decimals.

This digester is operating right at the proper removal range, which is between 50-60%.

Example:

Lab data from your 100,000 gallon primary anaerobic digester, which receives primary sludge only, is shown below. Using this data: Calculate the average volatile solids reduction, in percent.

Date Raw Sludge (%TS) Raw Sludge (%VS) Digested Sludge (%VS)

9/01 6.8 63.2 55.4

9/08 6.2 66.8 56.2

9/15 7.1 65.4 54.5

*The only two columns we need for calculating the volatile solids are the %VS. We do not need the raw sludge, %TS in our calculations. Remember, percents need to be displayed as a decimal. Also take into consideration that since three days are documented, you will need to take the average of the percent volatile solids first, and then plug those values into our formula.

Raw sludge (%VS) = 63.2 + 66.8 + 65.4 = 195.4 ÷ 3 = 65.1%

Digested sludge (%VS) = 55.4 + 56.2 + 54.5 = 166.1 ÷ 3 = 55.4%

Now plug those values into our volatile solids reduction formula (percents must be entered in decimal format):

As you can see, this digester is not operating at a healthy level, since the reduction level should be between 50-60%.

Summary

Untreated raw biosolids must be stabilized to minimize disposal problems. The stabilization of organic matter is accomlished biologically using a variety of organisms. These microorganisms convert the colloidal and dissolved organic matter into various gases and protoplasm. Since protoplasm has a specific gravity slightly higher than water, it can be removed from the treated liquid by gravity.

Assignment

Complete the math worksheet for this lesson and return to instructor via email, fax or mail.

Date	Raw Sludge (%TS)	Raw Sludge (%VS)	Digested Sludge (%VS)
9/01	6.8	63.2	55.4
9/08	6.2	66.8	56.2
9/15	7.1	65.4	54.5