Feasibility of Treating Wastewater Sludge with Chlorine Dioxide

By

Douglas Rittmann, Ph.D., P.E.

Water/Wastewater Consultant

 
 
Introduction
 

In March 1993, the USEPA established through Title 40 Part 503 the minimum standards for the use or disposal of sewage sludge. These standards address three principal sludge issues:  land-applied, distributed, or marketed sludge; disposal at dedicated sites or in sludge-only landfills (monofills); and incineration in sludge-only incinerators.  Also established were pollutant limits, management practices, and operational standards, for the final use or disposal of sewage sludge generated during the treatment of domestic sewage in a treatment works. In addition, the standards determine the monitoring frequency and recordkeeping requirements.  The states or permitting authority may impose more stringent requirements than USEPA standards. Generally, burial in landfills, either monofills or with municipal wastes, has been found to be the most cost-effective method of disposal. The feasibility of treating wastewater sludge with chlorine dioxide would depend upon its cost effectiveness in reducing pathogens as compared to lime stabilized treatment. 

 

Management Standards

 

Rule 40 CFR 503 establishes two new national standards. First, it sets standards for 10 heavy metals, pathogens (mainly disease-causing viruses, bacteria, and parasites), and emissions from incinerators.  Second, there is a standard for managing septage and sewage sludge and for their disposal.  Prescribed management practices were designed to limit human and ecological exposure to contaminants and then to ensure that the sludge produced is used on the land or is properly disposed of in a manner that protects both human health and the local environment.

 

General management practices include pollutant monitoring, pathogen reduction, vector attraction reduction, site restrictions, protection of threatened or endangered species, and record keeping.

 

Specific management practices for land application include the use of an agronomic rate of application based on the needs of site-specific crops; application in ways that prevent runoff to waters of the U.S. including a buffer zone of 10 meters; labeling or instructions for those who purchase sludge-derived products for individual use; and site restrictions.

 

Specific management practices for surface disposal methods include more stringent site restrictions, including those on grazing animals, crops, and human contact; certification or monitoring to ensure no contamination of groundwater; air monitoring for methane gas; and runoff collection requirements.

 

 

 

Toxic Metal Regulations

 

The 40 CFR 503 rule contains limits for 10 metal pollutants for land application: arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. The rule also specifies limits for 3 toxic metals for surface disposal: arsenic, chromium, and nickel.

 

Effect of Land Application

 

The new rule contains two regulatory strategies for land application depending on the quality of the sludge. Sludges that are shown or proven to be of “exceptionally high quality” (Class A) become exempt from further regulatory controls and can be used as freely as other soil amendments or fertilizer would be.  Sludges of good quality that do not meet the “exceptionally high quality” (Class B) standards can also be used on land if certain management practices are observed.

 

Pathogen and Vector Attraction Reduction

 

Pathogen and vector attraction reduction requirements are major changes from previous federal sludge regulations and include two classes of pathogen reduction: Class A and Class B. Class A is a Process to Further Reduce Pathogens (PFRP) standard; Class B is a Process to Significantly Reduce Pathogens (PSRP) standard.

 

There are differing requirements for Class A and Class B pathogens and vector attraction reductions as follows:

a.         Class A pathogen reduction. All options require pathogen reduction to indicate that the sludge has either: <1000 MPN fecal coliforms per gram total solids, or <3 MPN Salmonella per four grams of total solids; and one of the following six alternatives: control time and temperature, raise the sludge pH, reduce enteric viruses and helminth ova (low pathogen sludge), reduce enteric viruses and helminth ova (normal sludge), process to further reduce pathogens treatment, and process to further reduce pathogens equivalent treatment.

b.   Class B pathogen reduction.  There are three options for Class B pathogen reduction:  1).  <2,000,000 MPN coliforms per gram total solids (geometric mean of seven samples); PSRP (Process to Significantly Reduce Pathogens treatment; and PSRP equivalent treatment. Composting of sewage sludges is covered in the pathogen treatment processes.  When composting is practiced using either the in vessel composting method or the windrow composting method, the temperature of the sewage sludge is to be raised to 40 degrees C or 104 degrees F or higher and remain at 40 degrees C or 104 degrees F or higher for five days. For four hours during the five days, the temperature in the pile must exceed 55 degrees C or 131 degrees.  2). Five site restrictions also apply: A.  Food crops – no harvesting after sludge application for 14 to 38 months depending upon type of crop grown and how sludge is applied; B. Feed crops – no animal grazing for 30 days (low exposure areas; one year for high exposure areas; C. Turf – no harvesting for one year after sludge application; D.  Public access – restricted access for 30 days for low exposure areas; one year for high exposure areas; E.  Pasture—no animal grazing for 30 days after sludge application.

 

 

 3).   There are twelve methods of vector attraction reduction for land application, surface disposal and septage (see Table 1). Since the methods for vector attraction reduction are the same for land application or surface disposal, then the use of chlorine dioxide will depend upon its feasibility to reduce pathogens versus the lime stabilized sludge method in terms of meeting class A or B sludge requirements assuming that toxic metals’ levels and other analysis (total, fixed, and volatile solids; specific oxygen uptake rate) are in compliance too. In other words, since chlorine dioxide will only be able to reduce pathogen levels, its use will depend upon its cost effectiveness compared to stabilized lime treatment, which is currently considered the lowest cost alternative for pathogen reduction in sludge treatment. Therefore, what is a reasonable cost estimate for lime stabilized sludge treatment to meet Class A and Class B pathogen levels? 

 

Table 1 – Vector Attraction Reduction Methods

Method

Land Application

Surface Disposal

Septage

38% VS reduction

Yes

Yes

No

Bench test for low VS anaerobic sludge

Yes

Yes

No

Bench test for low VS aerobic sludge

Yes

Yes

No

Specific Uptake Oxygen Test (SOUR) <1.5 mg O2 /hr/g

Yes

Yes

No

14 days temp>40o C avg.Temp>45oC

Yes

Yes

No

PH> 12 for 2 hours & pH > 11.5 for 22.5 hours

Yes

Yes

No

75% dry solids(no primary treatment)

Yes

Yes

No

90% Dry Solids

Yes

Yes

No

Subsurface Injection

Yes

Yes

Yes

Incorporation

Yes

Yes

Yes

Daily Cover

Yes

Yes

Yes

PH > 12 for 30 minutes

No

No

Yes

 

Lime Stabilized Sludge Treatment

Excerpts from the paper by Paul Christy published at the December 1990 WPCF Conference in New Orleans, La presents “Process Equipment Considerations for Lime Staibilization Systems Producing PSRP (Process to Significantly Reduce Pathogens, Class B) and PFRP (Process to Further Reduce Pathogens, Class A) Quality Sludge”. This paper is based on experiences in the design and manufacture of over 25 sludge/lime stabilization systems and over 200 sludge cake conveying installations at municipal wastewater treatment plants. These experiences have led to general guidelines and suggestions. Each plant requires a unique system and each project should be reviewed and designed according to that plant's specific needs.

The system should be designed to raise pH to 12.0 and to hold pH at 12.0 for 2 hours. The same system can be used to meet the proposed class B pathogen reduction. A typical flow sheet for PSRP level of treatment is shown in Figure 1.

Equipment used for PFRP level of pathogen reduction must be designed to raise the pH to 12.0 and concurrently elevate the temperature to 70º C. This combination complies with EPA guidelines and is accepted by EPA as a PFRP process. Proposed 503 regulations require a Class A sludge which is treated by this method to demonstrate less than 100 indicator organisms per gram. Tests of sludges treated by this process have demonstrated the ability to consistently produce less than 100 indicator organisms per gram and as such would meet the proposed Class A requirements. A typical flow sheet for PFRP level of treatment is shown in Figure 2.

The most economical way to purchase lime is in bulk.  The lime storage equipment should be designed to hold at least 1-1/2 full truckloads and a maximum of one month's supply of lime. The storage equipment should protect the contents of the silo to prevent moisture from entering and from causing degradation of the lime. Figure 3 illustrates a recommended lime storage and feed system.

The use of lime for sludge stabilization can provide an economical and long-term solution for sludge disposal. A properly designed system can allow the flexibility to operate either a PSRP or PFRP process. A well-designed system can produce a PSRP (Class B) product for under $5 per ton of cake. A well- designed PFRP (Class A) system can operate as low as $10 per ton of sludge cake.  The lime cost is $75.00 per ton.

Chlorine Dioxide Sludge Treatment

Based on the above results, the feasibility of chlorine dioxide sludge treatment will depend upon its ability to meet Class A or Class B pathogen levels at costs less than $10 per dry ton or $5 per dry ton of sludge respectively.  Therefore, in table 2, a comparison table of costs for lime treatment versus maximum chlorine dioxide dosages at the breakeven point is based on one million gallons of liquid sludge at various % solids content. Please note that the chlorine dioxide cost is evaluated at $2.00 per pound, which could vary depending upon the market area and other factors. 

 

Table 2 – Breakeven Cost Comparison of Lime and Chlorine Dioxide

 

Sludge

Flow, MG

% Dry

Solids

T. Dry Tons

Lime Class A Cost

Lime Class B cost

*Max # of ClO2 for

Class A

*Max # of ClO2

Class B

 

Max. ppm of

ClO2

Class A

Max

ppm of

ClO2

Class B

1

4

167

$1670

$835

835

418

100

50

1

5

209

$2090

$1045

1045

523

125

63

1

6

250

$2500

$1250

1250

625

150

75

1

7

292

$2920

$1460

1460

730

175

88

* Chlorine Dioxide Cost based on $2.00 per pound and dry solids calculation based on 1.0 sp.gr.

 

Chlorine dioxide added to the sludge feed line prior to the dewatering process should permit optimum mixing and contact time for disinfection.  If chlorine dioxide is fed, in this manner, then the filtrate or centrate from the dewatering system will be more acidic and may lower the raw water pH when it is returned to the headworks (especially at low alkalinity levels). On the other hand, if the chlorine dioxide solution is applied directly to the cake (dewatered sludge) instead, then the final cake dryness would be decreased about 5% and the pH level of the centrate or filtrate is unaffected by the chlorine dioxide solution. If the final cake dryness is greater than about 18%, the dewatered sludge could be transported without loss from the truck. Dry lime addition requires exceptional mixing capability in order to assure adequate disinfection. On the other hand, liquid chlorine dioxide would be more soluble and penetrate the solids better permitting superior disinfection capability.

Another benefit of chlorine dioxide addition is the lower pH sludge could be ideal for use as a fertilizer as compared to the high pH lime sludge. Tim Smith of Washington State University states in an article about Soil pH, Fertilizers and Lime:  “Do NOT over-lime! Lime adjusts soil chemistry, it is not a fertilizer. A little too much can raise pH to undesirable levels and keep it there, causing serious management problems. Make certain you know how much lime is needed, then apply it over a number of seasons until your soil is back in balance.”  Therefore, the sludge pH with chlorine dioxide treatment would be in the neutral range and likely more ideal as a fertilizer than lime sludge.

In addition, it has been reported that the chlorite byproduct has bacteriostatic benefits in preventing bacterial re-growth problems within water distribution systems.  It is possible that the chlorite can also prevent bacterial re-growth from the sludge.  Ultimately, the more stable chlorite will further react to form chloride.

Conclusions

The following conclusions can be made with reasonable certainty:

1.      Chlorine dioxide can provide superior disinfection capability at economical cost as compared to lime.

2.      Chlorine dioxide can provide superior fertilizer quality sludge (Class A) at neutral pH levels as compared to lime.

3.      The chlorine dioxide solution should have better mixing and penetration of sludge solids than dry lime. 

4.      The chlorite by-product from chlorine dioxide addition should prevent bacterial re-growth and ultimately form chloride over time.

References

1.                  U.S. Government Printing Office via GPO Access, Title 40, Volume 27, CFR 503, Standards for the Use of Disposal of Sewage Sludge, (Revised as of July 1, 2003).

2.                  Christy, Paul G. “The Status of Municipal Sludge Management for the 1990’s”, WPCF Conference, New Orleans, La, Dec. 1990.

3.                  http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-501/c-10.pdf

4.                  http://www.ncw.wsu.edu/treefruit/soil/lime.htm