HANDLING AND DISPOSAL OF PROCESS WASTES

 

 

 

DISPOSAL OF PROCESS WASTES

 

Waste disposal of water treatment plant process wastes is an important consideration in the treatment Process and often overlooked. Solids removal technologies can produce several different types of liquid and solid wastes, including sludge, brine streams, backwash slurries, and spent media. Water systems must legally manage and dispose of process residuals in a manner that does not adversely impact human health and the environment.

 

Water systems should seek to reduce or eliminate the production of residuals if possible and should attempt to manage their treatment processes to avoid the production of hazardous waste.

 

 

 

 

 

TYPES OF WATER TREATMENT RESIDUALS

 

Water treatment plants produce four types of residuals:

 

 

 

 

VOLUMES OF WASTE SLUDGE PRODUCED

 

While all treatment processes will produce one or more types of residuals, the amount of residuals produced is a function of raw water quality, facility design and operating flow, the treatment process employed and the coagulant chemicals added. Processes such as lime and alum treatment produce an enormous amount of residual product. Lime plants generate 3 pounds of solids for every pound of hardness removed. Even small alum plants with average flows of .5 to 1 MGD can generate several 100,000 gallons of alum wastes per year that require disposal. For this reason it is sometimes beneficial to use polymers to reduce the amount of coagulant chemicals employed.

 

Although polymers are more expensive they are used in small concentrations typically below 1 PPM. Coagulant aluminum and iron metals required from 10 to 50 PPM and lime can be 150 to 200 PPM for hardness removal.

 

 

 

 

 

CONSTITUENTS IN WATER PLANT SLUDGE

 

Certain raw water characteristics can impact a systemís residual disposal options. One challenge for a water treatment plant is to optimize treatment while not creating residual disposal problems. Raw water characteristics that may impact disposal alternatives include:

 

 

 

 

DISPOSAL ALTERNATIVES FOR WATER PLANT SLUDGE

 

Water systems have the following disposal alternatives for the wastes they generate:

 

 

 

Disposal options will be limited by regulation, type of waste (liquid or solid), and the concentrations of contaminants in the waste. Most intermediate processing concentrates contaminants into a smaller volume for disposal, usually producing two waste streams: a liquid waste stream and a more concentrated semi-solid waste stream called sludge. Economics, disposal alternatives, and regulations will dictate the extent of intermediate treatment.

 

 

 

 

 

 

PROCESSING OF LIQUID SLUDGE

 

Sludge processing can greatly reduce the volume and the cost of handling of waste sludge. There are a wide variety of low and high tech options for the thickening of sludge. The choice of the option will depend on the local conditions and the cost. The following are some methods of thickening sludge and their efficiency ranges:

 

 

 

 

The following table shows the benefits derived from thickening a 2% sludge to various concentrations.

 

Concentration (% solids)

Volume Total (gallons)

Water Removed (gallons)

2%

10,000

0

4%

5,000

5,000

8%

2,500

7,500

16%

1,250

8,750

32%

625

9,375

64%

312

9,688

 

 

Thickening benefits can be very cost effective for large amounts of sludge that must be transported to off-plant sites.

 

 

 

 

 

 

SLUDGE DISPOSAL OPTIONS

 

Sludge residual disposal options include:

 

 

 

 

 

COMMERCIAL VALUE OF WASTE SLUDGE

 

Generally lime waste sludge has an agricultural value and can be spread on farmland. Application of commercial fertilizer results in a reduction in pH. Lime sludge works as an excellent conditioner. Polymer sludge can also be used as soil conditioner. Alum and Iron type sludge have no commercial value and must be disposed of.

 

 

 

 

 

 

TREATMENT AND DISPOSAL OF WATER REMOVED IN SLUDGE PROCESSES

 

Where sludge processing is used liquid wastes are produced that must also be disposed of. When possible, the most cost effective option is to recycle the liquid stream to the head end of the plant for reprocessing.

 

Current regulations require daily monitoring of liquid wastes prior to off-site disposal for water quality indicators such as pH, turbidity, TDS settleable solids and other harmful materials.

 

Treatment of liquid waste streams prior to off-site discharge often include: