Practical Manual of Wastewater Chemistry

Barbara Hauser

• Éditeur: CRC Press
• Année: 2018
• ISBN:
  9781351422598
• Édition: 1
• Sujet: Ciencias físicas
• Sous-sujet: Química

This manual is prepared to enhance the wastewater treatment plant operator's understanding of laboratory theory and procedure in the testing of wastewater, to appreciate the nature of the contaminants under analysis, the analytical methods, and their limitations.

1

Sampling and Laboratory Safety

Sampling

All laboratory testing depends first upon proper sampling technique. Sampling must be designed to obtain accurate data for identifying treatment process changes, and influent and effluent water qualities. Representative sampling is important. The objective is to remove a small portion which is representative of the entire flow, and adequately reflects actual conditions in the water.

Sample Location

The test requirements usually determine sample location (ex. raw, primary effluent, final). Take the sample where mixing is best, and the water is of uniform quality. Sampling location must be accessible; safe entry to confined spaces is imperative. Avoid slippery surfaces; do not climb on or under guardrails.

Composite Samples

This type of sample is taken to determine average conditions in a large volume of water whose chemical properties vary significantly over the course of a day. These are usually flow proportioned, and collected with an automatic sampler. Small aliquots are taken at regular intervals and pooled into one large sample over a 24 hour period. If composite samples are to be taken manually, the frequency of sampling should depend on the number of changes in water quality. Most often, grabs are taken hourly, and the volume of each is flow proportioned; samples are then combined into a composite for testing.

Grab Samples

A grab sample is taken all at once, at a specific time and place. In wastewater treatment grabs are usually taken at peak flow conditions. Bacteriologic samples, temperature and dissolved oxygen samples are always grabs. Insert container upside down into the water. Rotate open end toward direction of flow, and allow to fill under the surface. Sample about 12 inches underwater, or at center of the channel, about medium depth from the bottom. Avoid surface scum and bottom sediment. Be careful not to collect deposits from tank sidewalls. If large, uncharacteristic particles enter the sample, eliminate them. Fill sample bottle completely to exclude air space if sample is to be analyzed for DO, NH₃, H₂S, SO₂, pH, Chlorine Residual, Alkalinity or VOC’s. If preservative or dechlorinating agent has been added to empty sample bottle, adjust sampling technique so that the bottle does not overfill, or the chemical will be washed out. Be careful to handle sterile bottles for bacteriologic testing aseptically. Collect enough sample to allow duplicate and spiked analysis. Sludge samples are heaviest at the beginning of pumping, or from the bottom-most tap. Try for a representative portion of the flow.

If an influent and an effluent sample are desired from a process unit with a detention time of two hours, consider the timing. Take the influent sample now, and take the effluent sample in two hours. There will be a good chance of getting the same water.

The Sample Bottle

For chemical testing, the sample bottle must be clean. For bacteriologic testing, the sample bottle must be clean, and sterile. Bottles may be glass or plastic for most analyses; labels must be firmly attached to the sample bottle, not to the lid. Use labels that will not come off when damp. Use water insoluble ink pen. The label on a sample bottle should include sample ID number, date and time of collection, type of sample (raw, final), location (east aerator), adverse weather conditions, collector’s initials, analysis to be performed, sample preservation, if any.

Preservation and Transport

Dissolved oxygen, pH, temperature should be analyzed onsite, at the sampling location. All samples should be analyzed as soon as possible after collection.

Chain of Custody

This is a legal requirement, and refers to the recorded handling of a sample from collection to analysis. It allows for tracing individual samples in the case of a problem. The sample is in custody if it is in hand or in sight, if it is locked away or placed in a secure area where nobody can enter without the possessor’s knowledge. When the sample changes hands, a Change of Possession form is signed by both parties; date and time are recorded.

The Chain of Custody Records include:

• Sample Labels—for sample identification

• Sample Seals—for shipped samples, to ensure no tampering.

• Field Logbook—includes all information on label, container type, sample size, field analysis, number of samples taken.

• Chain of Custody Record—includes label information and Change of Possession forms.

Preservation Techniques and Holding Times

Test	Preservation	Max. Holding Time
Conform (Total & Fecal)	4 deg.C, .008% Na2S2G3	6 hrs.
Acidity	4 deg.C	14 days
Alkalinity	---	---
Ammonia	4 deg.C, H4SO4 to pH2	28 days
BOD	4 deg.C	48 hrs.
COD	4 deg.C, H2SO4 to pH2	28 days
Chloride	---	---
Chlorine Residual	---	do immed.
Color	4 deg.C	48 hrs.
Fluoride	---	28 days
Hardness	HNO3 to pH2	6 months
PH	---	do immed.
Metals	HNO3 to pH2	6 months
Mercury	HNO3 to pH2	28 days
Nitrate	4 deg.C	48 hrs.
Nitrite	4 deg.C	48 hrs.
Orthophosphate	4 deg.C. filter	48 hrs.
Total Phosphorus	4 deg.C. H2SO4 to pH2	28 days
DO	---	do immed.
Total Solids	4 deg.C	7 days
Dissolved Solids	---	48 hrs.
Total Suspended Solids	---	7 days
Settleable Solids	---	48 hrs.
Specific Conductance	---	28 days
Sulfate	---	---
Temperature	---	do immed.
Turbidity	4 deg.C	48 hrs.

Online Sampling and Analysis

There is increasing use of automatic online sampling and monitoring. It has the advantage of yielding constant and immediate results. Time is saved, and problems due to ...