Solids
Solid material in wastewater may be dissolved, suspended, or settled. Total dissolved solids or TDS (sometimes called filterable residue) is measured as the mass of residue remaining when a measured volume of filtered water is evaporated. The term total suspended solids (TSS) refers to the nonfilterable residue that is retained on a glass‐fiber disc after filtration of a sample of wastewater. Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour. Turbidity is a measure of the light‐scattering ability of suspended matter in the water. Salinity measures water density or conductivity changes caused by dissolved materials.
Table 3.5 General pollution of wet process for starch producing.
| Raw material | Volume of water (m3/T) | BOD5 (kg/T) |
| Corn starch | 2–4 | 5–12 |
| Rice starch | 8–12 | 5–10 |
| Wheat starch (gravity separation) | 10–12 | 40–60 |
Table 3.6 Potato processing wastes.
| Facility or shop | Volume of water (m3/T) | SS (kg/T) | BOD5 (kg/T) |
| Preparation | |||
| Transport and washing | 2.5–6 recyclable | 20–200 | — |
| Peeling and cutting | 2–3 | — | 5–10 |
| Flakes | |||
| Bleaching and cooking | 2–4 | — | 10–15 |
| Crisps | |||
| Bleaching | 2.2–5 | 5–10 | 5–15 |
| Starch extractiona | |||
| Washing, grating, grinding | 2–6 (Red water) | Recyclable pulp | 20–60b |
| Pressing – refining | 1 |
a International Starch Institute Aarhus Denmark Potato Starch Effluents.
b Including preparation water.
Oxygen
Most aquatic habitats are occupied by fish or other animals requiring certain minimum dissolved oxygen concentrations to survive. Dissolved oxygen concentrations may be measured directly in wastewater, but the amount of oxygen potentially required by other chemicals in the wastewater is termed as oxygen demand. Dissolved or suspended oxidizable organic material in wastewater will be used as a food source. Finely divided material is readily available to microorganisms whose populations will increase to digest the amount of food available. Digestion of this food requires oxygen, so the oxygen content of the water will ultimately be decreased by the amount required to digest the dissolved or suspended food. Oxygen concentrations may fall below the minimum required by aquatic animals if the rate of oxygen utilization exceeds replacement by atmospheric oxygen.
Basically, the reaction for biochemical oxidation may be written as
Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:
Biochemical and Chemical Oxygen Demands
The most widely used parameter of organic pollution applied to both wastewater and surface water is the five‐day biochemical oxygen demand (BOD5).
This determination involves the measurement of the dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter in a five‐day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the ultimate BOD. Because the ultimate BOD is so time‐consuming, the BOD5 has been almost universally adopted as a measure of relative pollution effect.
COD
COD is widely used to characterize the organic strength of wastewaters and pollution of natural of natural waters. The test measures the amount of oxygen required for chemical oxidation of organic matter in the sample to carbon dioxide and water.
Both the BOD and COD tests are a measure of the relative oxygen‐depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants, whereas the COD test measures the oxygen demand of biodegradable pollutants plus the oxygen demand of non‐biodegradable oxidizable pollutants.
Nitrogen
Nitrogen is an important nutrient for plant and animal growth. Atmospheric nitrogen is less biologically available than dissolved nitrogen in the form of ammonia and nitrates. Availability of dissolved nitrogen may contribute to algal blooms. Ammonia and organic forms of nitrogen are often measured as total Kjeldahl nitrogen, and analysis for inorganic forms of nitrogen may be performed for more accurate estimates of total nitrogen content.
Phosphates
Phosphates enter the water ways through both NPSs and point sources. NPS pollution refers to water pollution from diffuse sources. NPS pollution can be contrasted with point source pollution where discharges occur to a body of water at a single location. The NPSs of phosphates include natural decomposition of rocks and minerals, storm water runoff, agricultural runoff, erosion and sedimentation, atmospheric deposition, and direct input by animals/wildlife, whereas point sources may include wastewater treatment plants and permitted industrial discharges. In general, the NPS pollution typically is significantly higher than the point sources of pollution. Therefore, the key to sound management is to limit the input from both point sources and NPSs of phosphate. High concentration of phosphate in water bodies is an indication of pollution and largely responsible for eutrophication.
Pollutant Concentration and Loading in Wastewater
Loadings on wastewater treatment units are often expressed in terms of pounds of BOD or TSS per day or pounds of solids per day, as well as quantity of flow per day. The relationship between the parameters of concentration and flow is based on the following conversion factors: 1.0 mg/l, which is also 1.0 parts per million (ppm) by weight, equals 8.34 or 62.4 lb/million gal (MG), since 1 gal of water weighs 8.34 lb.
Conversion of mg/l (ppm) to lb/MG
or
(3.2)
where
- A = BOD, TSS, or other constituents (mg/l)
- Q = volume of wastewater (MG or million ft3)
- 8.34 = (lb/MG)/(mg/l)
- 62.4 = (lb/million ft3)/(mg/l)
EXAMPLE 3.1
You recorded 4.0 ppm of TSS in an industrial wastewater flowing into Wisconsin River at a rate of 25 million gallons per day (MGD). Find out how many pounds (lb) of TSS are entering into Wisconsin River per day.
SOLUTION
EXAMPLE 3.2
Industrial wastewaters have a total flow of 2 930 000 gpd, BOD of 21 600 lb/day, and suspended solids of 13 400 lb/day. Calculate the BOD and SS concentrations.
SOLUTION
From the relationship in Eq. (3.1),
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