Differences between Sewage Sludge, Medical Wastes, 

Hazardous Wastes and Municipal Solid Wastes and 

the Incinerators Used to Combust these Materials 

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A. Sewage Sludge 

Sewage sludge (biosolids) is defined as a solid, semi-solid, or liquid residue generated 

during the treatment of domestic sewage in a publicly owned treatment works (POTW), 

and consists of solids removed in primary, secondary, or advanced wastewater treatment 

processes and any material derived from sewage sludge (e.g., blended sewage 

sludge/fertilizer product). 

Domestic sewage is generated by residences, businesses, etc., located in the POTWs 

services area, along with Stormwater runoff, and is transported to the POTWs via sewers. 

All of the sewage is treated at POTWs which are owned by municipalities or other 

governmental agencies, as a service to its rate payers. 

The land application, surface disposal and incineration of sewage sludge is regulated 

under 40 CFR Part 503 (Use or Disposal of Sewage Sludge). 

 

Sewage sludge incinerators are regulated under Subpart E of the Part 503 Regulation, 

which established pollutant limits, operational standards and recording, monitoring & 

reporting requirement that ensure protection of human health and the environment. This 

includes, but is not limited to: 

- Site specific emission limits for arsenic, cadmium, chromium, lead and nickel 

- Compliance with the NESHAP limits for beryllium and mercury 

- Compliance with an operational standard for total hydrocarbon or an alternative 

standard for carbon monoxide 

Sewage sludge incinerators are also regulated under 40 CFR Part 60, Subpart O (New 

Source Performance Standards for Sewage Sludge Treatment Plants) and Section 112 of 

the Clean Air Act (Hazardous Air Pollutants). 

There are two types of incinerators that are used to combust sewage sludge (i.e., multiple 

hearths and fluidized beds). 
It was estimated that 850,000 - 1,000,000 dry metric tons of sewage sludge is incinerated 

annually in 231 sewage sludge incinerators located at 124 POTWs in 24 states and Puerto 

Rico. However, the amount of sewage sludge being incinerated has been increasing, due 

to public/local opposition to land application programs. 

B. Medical Waste

According to EPA’s Economic Impact Report on Revised MACT Standards for 

Hospital/Medical/Infectious Waste Incinerators (dated October 2008), medical wastes 

come from hospitals, laboratories, clinic/outpatient care, physicians’ offices, dentist 

offices, veterinarians, long-term health care facilities, free-standing blood banks and 

funeral homes. Medical wastes consist of: 

• Cultures and Stocks (infectious agents) 

• Pathological and chemo wastes (organs, tissues, body parts, fluids, etc) 

• Human blood and blood products 

• Sharps (needles, syringes, blades, vials, tubing, etc.) 

• Animal wastes 

• Isolation wastes (waste from patients/animals with highly 

communicable diseases) 

In 2008, there were a total of 57 medical waste incinerators in service in the United States 

that combusted 146,236 tons of medical waste annually. The following is a breakdown 

of the industry by type owning medical waste incinerators: 
Category Number of Incinerators Share of Waste (%) 

Hospital 31 20.5 

Commercial 14 75.5 

Pharmaceutical 4 2.8 

University 4 0.3 

Government Research 2 0.2 

Military Installations 2 0.7 

Total 57 100 

There are three types of incinerators are being used to combust medical wastes (i.e., 

starved air, excess air and rotary kiln.) 

 Starved Air Excess Air 

 

 

 Rotary Kiln

There are four categories of MACT Standards for medical waste incinerators, based on 

the feed throughput of the incinerators: 
- Large: >500 lbs/hour 

- Medium: >200 to < 500 lbs/hour 

- Small: < 200 lbs/hour 

- Small Rural: < 2,000 lbs/week 

Within these categories, there are MACT Standards for medical waste incinerators based 

on when the unit was constructed: 

• For which construction was commenced on or before June 20, 1996 or for which 

modification commenced on or before March 16, 1998 (§ 60.323(a)(1)) 

• For which construction was commenced after June 20, 1996 but no later than 

December 1, 2008, or for which modification is commenced after March 16, 1998 

but not later than April 6, 2010 (§ 60.32e(a)(2)). 

EPA estimated the cost of complying with the MACT Standards (Total MACT Floor 

Costs) to be $21,125,952 or $370,000 per incinerator. 

C. Hazardous Waste 

Hazardous wastes are regulated under the Resource Conservation and Recovery Act 

(RCRA) Subtitle C and are either specifically listed as hazardous waste or are considered 

hazardous wastes because they exhibit one of the four characteristics (ignitability, 

corrosivity, reactivity, or toxicity. 

According to U.S. EPA’s 2007 National Biennial RCRA Hazardous Waste Report, 

3,047,982 tons of hazardous wastes were incinerated at 140 hazardous waste combustion 

facilities in 2007, containing approximately 260 combustors. Hazardous wastes are 

typically combusted in incinerators, boilers and industrial furnaces. 

Below is a typical cross-section of a hazardous waste incinerator: 
On September 20, 1999, EPA promulgated MACT Standards for Hazardous Waste 

Combustors. Due to a request for a judicial review of the rule and challenges to emission 

standards and implementation procedures, the DC Circuit Court vacated the rule and new 

Standards were promulgated on October 12, 2005 and published in the Federal Register 

on December 19, 2005. 

Following promulgation, EPA received petitions for reconsideration. The October 12, 

2005 rule was amended on April 8, 2008 and a reconsideration final rule notice was 

published in the Federal Register on October 28, 2008. 

We were unable to find the capital costs to modify the existing municipal hazardous 

waste incinerators to meet the MACT Standards or the annual increase in operating & 

maintenance costs. 

D. Municipal Solid Waste 

Municipal Solid Waste (i.e., trash or garbage) is made up of things that we commonly use 

and then throw away (e.g., containers and packaging, durable and non-durable goods, 

yard trimmings, food scraps and other wastes). 

According to EPA, 250 million tons of municipal solid waste were generated in the 

United States in 2008. 

EPA estimates that 31 million tons (12.6%) of municipal solid waste were combusted 

with energy recovery. However, municipal solid waste that was combusted without 

energy recovery is not separately broken out. Instead it is included with the amount that 

is landfilled and the combined total is 135 million tons (54%). 

The following is a summary of the large municipal solids waste combustors (having a 

capacity of greater than 250 tons per day) in the U.S. in 1995 and 2000. 

Year Number of Large 

MWC Plants 

Number of Large 

MWC Units 

Capacity 

(tons/day) 

1995 63 164 88,652 

2000 66 167 89,477 

The following is a summary of the small municipal solid waste combustors (having a 

combustion capacity of 25 - 250 tons) in the U.S. in 1990, 1995, 2001 and 2005: 

Year Number of Small 

MWC Plants 

Number of Small 

MWC Units 

Capacity 

(tons/day) 

1990 67 149 14,854 

1995 54 117 10,965 

2001 39 84 8,305 

2005 26 60 7,172 
Mass burn combustors are commonly used to incinerate municipal solid waste. They 

include mass burn-waterwall, mass burn- refractory wall, and mass burn-rotary waterwall 

combustion units. Some MWC utilize refuge-derived fuels, while there still could be a 

few starved air units. 

The following are cross-sections of the various types of municipal solid waste 

combustors. 

 Mass burn-waterwall 

 Mass burn- rotary waterwall 
 Mass burn-refractory wall with rotary/grate kiln 

 

Refuge derived fuel –spreader stoker boiler 
 Starved air combustors with transfer rams 

We were unable to find the capital costs to modify the existing municipal solid waste 

incinerators to meet the MACT Standards or the annual increase in operating & 

maintenance costs. 

RPD 

12-2-09