Septage

Septage is the liquid and solid (scum and sludge) waste produced in individual on-site wastewater disposal systems such as septic tanks and cesspools and is approximately 90-98% water. Septage periodically is removed from septic tanks and cesspools during cleaning operations and is often discharged to wastewater treatment plants for disposal and treatment.

Periodic removal or pumping of septage is essential to the long-term operation of septic tanks (Figure 18.l) and septic systems. Once removed, septage disposal occurs. Septage disposal practices vary across the nation and even within states. However, more uniform requirements for septage disposal practices are being adopted nationwide. Examples include the adoptions of new regulations (40 CFR503) in 1993 and federal guidelines for septic systems in 2002 by the U.S. Environmental Protection Agency.

There are several methods that are available for the disposal of septage. These methods incorporate the treatment of septage to control malodors, reduce solids volume, decrease the strength of pollutants, and destroy pathogens (disease-causing agents). The major methods available for the disposal of septage include (1) co-disposal with solid wastes, (2) co-treatment with wastewater, (3) land application, and (4) processing at separate facilities (Table 18.1).

For several reasons the co-treatment of septage with wastewater is a popular method for the disposal of septage (Table 18.2).This method is used at many waste-water treatment plants. However, small wastewater treatment plants are more vulnerable to "upset" conditions from the discharge of septage than large wastewater treatment plants, if septage is not properly discharged to the plants. Problems of organic overloading can be overcome by collecting and storing septage during the daytime hours and then discharging septage to the wastewater treatment plant when

Wastewater Organisms

Connected to house sewer

Access port

PVC vault -

PVC intrusion pipe

Effluent to disposal field

FIGURE 18.1 Septic tanks.

TABLE 18.1 Methods Available for the Disposal of Septage

Co-disposal with solid wastes Composting Landfilling Co-treatment with wastewater Biological treatment Chemical treatment Land application Subsurface application Surface application Processing at separate facilities Biological treatment Chemical oxidation Composting Lime stabilization

TABLE 18.2 Reasons for the Co-treatment of Septage with Wastewater

Cooperation with regulatory agencies or political bodies

Only method available for disposal

Revenue for wastewater treatment plants

Use of excess treatment plant capacity

Year-round availability for septage haulers

TABLE 18.3 Significant Components in Domestic Wastewater and Septage

Component

Range in Concentration (mg/liter)

Typical Concentration (mg/liter)

Domestic Wastewater

Septage

Septage

bod5

110-400

2,000-6,000

3,000

COD

250-1,000

5,000-80,000

30,000

Grease

50-150

5,000-10,000

8,000

NH+

12-50

100-800

400

Phosphorus

4-15

50-800

250

TKN

8-35

100-1,600

700

TSS

100-350

2,000-100,000

15,000

VSS

80-275

1,200-14,000

7,000

the influent organic loading is lowest. Biological treatment of septage at wastewater treatment plants usually is achieved in the activated sludge processes.

Septage is approximately 50 times as concentrated as domestic wastewater. Significant biological, chemical, and physical components of septage vary greatly (Table 18.3). Of the total nitrogen in septage, approximately 80% is organic nitrogen (TKN) and 20% is ionized ammonia (NH4+). Compared to those of wastewater, carbon (cBOD)-to-nutrient (nitrogen and phosphorus) ratios for septage are low, but not below levels that tend to inhibit acceptable degradation of cBOD. Concentrations of metals such as copper, lead, and zinc vary greatly in septage.

Variations in the concentrations of the components in septage occur for several reasons. The largest range in values of components often is found in communities that do not regulate the collection and disposal of septage. Variations in the concentration of metals and minerals in septage is due in large part to the potable water supply used and discharged to septic tanks.

Other components of septage that are of interest to wastewater treatment plant operators include grit, pathogens, foam, odor, and color. Septage contains relatively high levels of grit and pathogens, especially protozoan cysts and oocyts and helminth (worm) eggs. Septage is difficult to dewater and may foam easily upon agitation. Septage is malodorous, and the offensive odor associated with septage is produced through the anaerobic degradation of cBOD and the production and release of volatile fatty acids, nitrogen-containing compounds, and sulfur-containing compounds (Table 18.4). Color is a component of septage. Wastewater contaminated with septage becomes black.

The quantity of sludge and scum in septage also varies greatly and is affected by several factors. These factors include whether a kitchen food grinder (disposal unit) is used, the quantity of oil and grease discharged to the septic tank, and the

TABLE 18.4 Most Commonly Occurring Malodorous Compounds Produced through Anaerobic Degradation of BOD

Group

Compound

Chemical formula

Odor

Acid

Acetate

CH3COOH

Vinegar

Butyrate

CH2(CH3)2COOH

Rancid

Valeric acid

CH2(CH3)3COOH

Sweat

Nitrogen-containing

Ammonia

NH3

Irritating

Dimethylamine

(CH3)2NH

Fish

Indole

C8H6NH

Fecal

Methylamine

CH3NH2

Rotten fish

Skatole

C9H8NH

Fecal

Sulfur-containing

Ethylmercaptan

C2H5SH

Rotten cabbage

Hydrogen sulfide

H2S

Rotten egg

Methylmercaptan

CH3SH

Cabbage

Methylsulfide

(CH3)2S

Rotten vegetables

Methyldisulfide

(CH3)2S2

Putrefaction

TABLE 18.5 Significant Components of a Properly Designed Septage Storage Tank

Adequate storage capacity to compensate for potential treatment plant problems

All-weather access for easy unloading of septage

Design considerations for minimization of malodor release

Fencing to prevent unauthorized entrance

Landscaping to reduce the visual impact of the storage tank

Safety signs to indicate potential danger of drowning and hazardous gases

Water-tight construction to prevent septage leaks frequency of septage tank pumping. The frequency of pumping influences the degree of bacterial digestion of solids within the septic tank. The longer the digestion time that is provided, the smaller the quantity of sludge and scum that is produced.

Septage should be safely discharged to a wastewater treatment plant. Usually, septage is added to the influent wastewater or sludge. When added to the influent wastewater, septage should be slowly blended with the wastewater to minimize its impact upon the treatment process. When added to sludge, septage should be screened and slowly added to sludge for stabilization and dewatering. Whenever septage is added to wastewater influent or sludge, the feed rate of septage should be controlled.

Septage may be added to a wastewater treatment plant by discharging the septage through a bar screen to a receiving tank (Table 18.5). The septage is then slowly discharged to the treatment plant from the receiving tank at a constant rate. At some wastewater treatment plants, septage may be slowly discharged to a designated manhole. As the septage travels through the sewer system, it is mixed and is diluted with raw wastewater. The daily volume of septage received at a waste-water treatment plant should be closely regulated to prevent overloading of the treatment process. Limiting the hours for receiving septage can help to prevent overloading.

Problems associated with overloading from septage often occur at small waste-water treatment plants with limited capacity. These problems can be overcome by collecting and storing the septage during the daytime hours and then discharging the septage when loading conditions are favorable.

Receiving tanks often experience problems with spills, debris, and odors. To overcome these problems, a water supply with a hose should be provided at the receiving tank to clean the septage truck unloading area. A dumpster may be provided for the disposal of debris.

Malodor control can be achieved by increasing the pH of the septage, adding appropriate bioaugmentation products (bacterial cultures), or treating septage with a strong oxidant. The pH of septage may be increased with the use of hydrated lime (calcium hydroxide, Ca(OH)2) or quicklime (calcium oxide, CaO). Cultures of Pseudomonas may be added to septage to control malodors. Species of Pseudomonas easily degrade sulfur-containing compounds that are associated with malodor production.

The addition of a strong oxidant to septage can not only decrease malodors but also reduce many of the undesired features of septage. Oxidants commonly added to septage include sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), ozone (O3), and potassium permanganate (KMnO4). In some cases the addition of a strong oxidant may enhance malodor production.

The addition of a strong oxidant may reduce the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) associated with septage. The oxidant may promote biological degradation of some refractory organic compounds in septage, improve settleability and dewaterability of septage, and destroy pathogens. Also, nitrite may be oxidized to nitrate, and sulfides may be oxidized to sulfate or elemental sulfur.

Strong oxidants break some bonds in large complex molecules to form smaller, more easily degradable molecules and eliminate many large water-bound colloidal molecules. These actions improve biological degradation of organic compounds and improve solids settleability and dewaterability.

Unless hydrogen sulfide (H2S) is present in relatively high quantities, it does not contribute to a malodorous condition. Sulfides (HS-) combine with the metals in septage solids to form insoluble metallic sulfides.

The source of septage also is a concern to wastewater treatment plant operators. Septage should not, but may, contain industrial wastes, grease-trap wastes, or toxic wastes, and the wastewater treatment plant may not be prepared to handle these wastes. Therefore, the wastewater treatment plant should have a contract with all septage haulers who are approved to discharge septage to the wastewater treatment plant or sewer system (Table 18.6).

TABLE 18.6 Suggested Septage Hauler Requirements

Apply for and be issued a septage discharge permit prior to discharge and/or use of the wastewater treatment plant or sewer system. Have proof of liability insurance with coverage limits as required. Possess a valid septage hauling permit.

Provide indemnity bond, deposit, or other payment guarantee sufficient to guarantee payment fees. Record and submit the source of septage discharged to the wastewater treatment plant or sewer system.

Wastes shall only be accepted for treatment, if the wastewater treatment plant processes and final effluent are not adversely affected.

The discharge and treatment of septage at wastewater treatment plants presents several operational concerns. These concerns include increased operational costs, release of malodors, growth of undesired filamentous organisms, and unacceptable wastes, especially toxic wastes.

Increased operational costs occur through increased sludge production and disposal and increased dissolved oxygen demand to satisfy the degradation of BOD. The degradation of BOD includes carbonaceous BOD and nitrogenous BOD. An appropriate discharge or treatment fee should be charged and collected from each septage hauler. This fee usually is based on gallons of septage discharged.

In order to protect the wastewater treatment plant against industrial wastes, grease-trap wastes, toxic wastes, or other unacceptable wastes, a small sample from each truckload of septage should be collected, labeled, and refrigerated for one to two weeks. If the wastewater treatment plant becomes upset after the discharge of septage to the treatment process, the collected samples can be analyzed to determine if the septage is responsible for the upset. A representative sample of septage may be obtained by collecting a small and flow-proportioned quantity of septage at the beginning, mid-point, and end of the septage flow stream to the receiving tank.

When discharged to an activated sludge process excess, quantities of sulfides and septic waste (short-chained acids and alcohols) promote the rapid and undesired growth of filamentous organisms such as Beggiatoa spp., Thiothrix spp., and type 021N. These organisms in undesired numbers contribute to settleability problems and loss of solids. When discharged to any biological wastewater treatment process, toxic wastes contribute to upset through inhibition of bacterial and protozoan activities.

Was this article helpful?

0 0
Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

Get My Free Ebook


Post a comment