Common Issues with Groundwater Exceeding Standards and a Detailed Introduction to Treatment Equipment

Groundwater is water stored in the pores of geological strata below the vadose zone, including rock pores, fissures, and cavities. Due to the varying regions through which it flows, groundwater often contains elevated levels of certain substances.

Sources of Pollution:
① Domestic sewage and household waste can increase the total mineralization, total hardness, nitrate, and chloride levels in groundwater, and may sometimes cause pathogen contamination.
② Industrial wastewater and industrial waste can increase the concentration of organic and inorganic compounds in groundwater.
③ The application of chemical fertilizers and manure in agriculture can cause widespread increases in nitrate levels in groundwater. Pesticide contamination of groundwater is generally milder and occurs primarily in shallow aquifers. Agricultural cultivation activities can promote the oxidation of soil organic matter, such as the conversion of organic nitrogen into inorganic nitrogen (primarily nitrate nitrogen), which then enters the groundwater through seepage. Natural saline water can also contaminate natural freshwater aquifers.

Characteristics of groundwater with excessive iron content:
1. Well water turns yellow; clear well water turns pale yellow after sitting in a basin for several hours to one day;
2. Well water contains yellow sediment; after a few days, yellow iron sludge settles at the bottom of the basin, and the basin walls become stained yellow;
3. After boiling, the water turns slightly yellow; the higher the iron content, the deeper the yellow color;
4. Well water has an oily film; an oily film floats on the water surface;
5. The well water has a metallic odor; freshly drawn well water has a distinct iron-like metallic odor.
6. Manganese has a weaker staining ability; when iron levels are also excessive, the staining effect of manganese is not noticeable;
7. When manganese levels are excessive on their own, it stains the tank walls and the floor where water is used black and may cause peeling;
8. Washing clothes with this water causes the colors to fade.

Characteristics of groundwater with excessive manganese:
1. Manganese has a relatively weak staining ability; when iron levels are also excessive, the staining effect of manganese is not noticeable;
2. When manganese levels are excessive on their own, it will stain the walls of tanks and the floors where water is used black, and peeling may occur;
3. Laundry will cause the colors of clothes to darken.

Characteristics of groundwater with excessive hardness
1. After boiling, white lumps form at the bottom of the kettle;
2. Water pipes become clogged, causing poor water flow;
3. White residue remains on furniture after cleaning.
To address the above groundwater contamination issues, a groundwater iron and manganese removal system can be used. After treatment, the iron, manganese, and hardness levels in the groundwater will all meet normal standards.

Causes of Excessive Total Hardness in Groundwater

Question 1:
Why Does Total Dissolved Solids in Groundwater Exceed Standards? 5 points
Groundwater refers to water stored in the pores of rocks beneath the ground surface; in a narrow sense, it refers to water in saturated aquifers below the groundwater table. According to the national standard “Hydrogeological Terminology” (GB/T 14157-93), groundwater refers to all forms of gravitational water buried beneath the ground surface.
Total Dissolved Solids:
Formerly known as total mineralization. This refers to the total amount of dissolved components in water, including the total mass of various ions, molecules, and compounds dissolved in water, but excluding suspended solids and dissolved gases.
Mineralization is expressed in grams per liter (g/L). Generally, mineralization is determined by heating one liter of water to 105–110°C until all the water has evaporated; the mass of the remaining residue is the water’s mineralization. It can also be calculated by summing the concentrations of various ions in the water and subtracting half the HCO₃ content. Groundwater is generally classified by salinity (M) as follows: Freshwater, M ≤ 50 g/L. The types of major salts present in groundwater often vary with changes in salinity.
Exceeding the standard for total dissolved solids (TDS) in groundwater indicates that the TDS content is relatively high, surpassing the required limit. For example: slightly brackish water, M = 1–3 g/L

Question 2: What are the hazards of high-hardness water?
1. Long-term consumption of high-hardness water can lead to disorders in the cardiovascular, nervous, urinary, and hematopoietic systems.
2. Boiled water tastes unpleasant and often causes scale buildup at the bottom of kettles. This severely affects the taste and quality of food.
3. When bathing, hair and skin often feel dry and tight, which damages the skin and accelerates aging.
4. When washing clothes, detergent is wasted, and clothes are difficult to clean thoroughly. After washing, clothes become brittle and stiff, and retain a detergent odor.
5. Water stains and spots frequently appear on tableware and bathroom fixtures, requiring frequent cleaning; limescale may even form on sinks and walls.
6. As water heaters age, their thermal efficiency decreases due to accumulated limescale. This not only wastes energy but also creates safety hazards.

Question 3: Is groundwater hard?
----Yes. Because groundwater remains underground year-round, it dissolves many minerals, so its hardness is generally high.

Question 4: The groundwater hardness is too high, with a total hardness of around 10,000. How should it be softened? Try calcium and magnesium ion removers.
For example, anionic polyacrylamide (PAM) can be used to remove calcium and magnesium.
Currently, the most established technical methods for separating and removing sulfate ions include the barium chloride method, calcium chloride method, freezing method, barium carbonate method, ion exchange method, and membrane separation method.

Iron and manganese removal filters are also known as manganese sand filters.
Iron and manganese removal filters are common equipment in groundwater treatment. Iron and manganese are almost always present together in groundwater, particularly in northern China and regions rich in iron and manganese ores. When used as process water in various chemical industries—such as papermaking, textiles, printing and dyeing, and leather processing—high levels of iron and manganese can severely affect product quality. When iron content is high, the water develops an iron-like odor, which affects its taste. As for drinking water, China’s “Sanitary Standards for Drinking Water” (GB5749-85) stipulates that iron content must be ≤0.3 mg/L and manganese content ≤0.1 mg/L. Raw water exceeding these standards must undergo iron and manganese removal treatment. Long-term consumption of water with excessively high iron and manganese levels can also seriously affect human health. I. Methods for Removing Iron and Manganese Water with excessively high iron and manganese content is typically treated by oxidizing dissolved divalent iron or divalent manganese into insoluble trivalent iron or tetravalent manganese compounds. Manganese sand filters can effectively remove iron and manganese through adsorption and filtration. The chemical reactions occurring during the oxidation process are as follows: 4Fe²⁺ + O₂ + 10H₂O = 4Fe(OH)₃ + 8H⁺; 2Mn²⁺ + O₂ + 2H₂O = 2MnO₂ + 4H⁺

II. Oxidation Methods for Iron and Manganese Removal
1. Jet aeration: Generally used in projects with low flow rates and moderate iron and manganese levels. The advantage is lower construction costs.
2. Niconi Pump: Similar in principle to the jet aerator, this method primarily relies on the Niconi pump’s large circulation to draw in air for oxidation.
3. Tower Aeration: Generally used in projects with high flow rates and high iron and manganese content. The oxygen supplied by the blower comes into full contact with the water inside the tower, which is filled with multi-faceted hollow balls to increase the water’s specific surface area.
4. Blower aeration is generally used for high-flow applications, such as municipal water supply, where iron and manganese levels are high. It involves higher construction costs and higher operating expenses.

III. Structure of Iron and Manganese Removal Filters

These are typically constructed from carbon steel or 304 stainless steel, with an internal coating of epoxy resin or a natural rubber lining for corrosion protection. The water distribution system includes basket-type distribution, dome-shaped plate distribution, stainless steel pipe distribution, and flat plate distribution. Water distribution caps include ABS mushroom-type and wire-wound cylindrical designs. For tubular and dome-plate distribution systems, a bedding layer composed of large river pebbles or coarse quartz sand is placed beneath them. This is primarily due to their high porosity and low resistance, which facilitates water collection and backwashing. Scope of Application: Primarily used in the food, beverage, paper, and brewing industries for treating water with excessive iron content; for iron removal from groundwater and well water intended for drinking; and for geothermal engineering and swimming pool recirculating water systems.

Product Features:
Stable effluent water quality, high efficiency in iron and manganese removal, and low operating and maintenance costs. Compared to natural oxidation methods for iron removal, it does not require large-scale reaction and sedimentation structures, resulting in a small footprint.
Technical Parameters of Iron and Manganese Removal Filter
1. Inlet Water Quality
Iron content: ≤20 mg/L Manganese content: ≤3 mg/L Inlet turbidity: <15 mg/L Alkalinity: ≤2 mg/L pH: >5.5
2. Water temperature: 6–10°C Operating pressure: <0.4 MPa Operating temperature: Ambient
3. Operating Parameters
Filtration Rate: 7–15 m/h Filter Bed Height: 800–1200 mm Compressed Air Pressure: 1–2 kg/cm² Backwash Air Flow Rate: 18–25 L/m²·s Backwash Duration: 5–10 minutes Backwash Intensity: 15 L/m²·s (single-layer), 12 L/m²·s (double-layer)
Working Principle: The iron and manganese removal filter utilizes an oxidation process to convert divalent iron and manganese ions in water into trivalent iron and manganese ions, which are then removed through adsorption filtration, thereby reducing the iron and manganese content in the water. The iron and manganese removal filter employs processes such as jet aeration using residual pressure from the well pump, tubular mixing for oxygenation, disc-type water distribution for degassing, and contact oxidation filtration within the filter bed. This design moves the external aeration and oxidation processes of traditional equipment into the unit itself, allowing the system to operate solely on the residual pressure of the well pump. It offers significant advantages including low energy consumption, a simple process, stable performance, and reduced overall investment costs.

1. Aeration Method:
This process serves two purposes: first, to increase dissolved oxygen in the water; second, to remove CO₂ to raise the water’s pH, thereby oxidizing divalent iron into trivalent iron precipitates, which are then filtered out.

2. Filtration:
This process serves two purposes: first, to remove flocs formed by trivalent iron; second, to catalyze the oxidation of most remaining unoxidized divalent iron and facilitate ion exchange through peroxide oxidation, thereby achieving iron removal. The iron sludge produced after filtration can be recovered and reused.
The filter media used include quartz sand and natural manganese sand. The former is suitable for raw water with iron content below 4 mg/L and a pH above 6.8; the latter is suitable for raw water with iron content below 20 mg/L and a pH above 6. The principle of manganese removal is the same as that for ferrous iron removal.

IV. Guidelines for Selecting Iron and Manganese Removal Filters

1. Equipment Selection
① Depending on water quality, a single-stage or two-stage treatment system may be selected; depending on water flow, a single unit or multiple units in parallel may be chosen.
② The process flow for iron and manganese removal should be determined based on the following conditions:
a. When the raw water contains ≤10 mg/L of iron and ≤0.5 mg/L of manganese, a single-stage treatment system should be used; when the iron content is <20 mg/L or the manganese content is >1 mg/L, a two-stage iron and manganese removal system should be used.
b. When the iron content in the raw water is ≤2.0 mg/L and the manganese content is ≤1.5 mg/L, the following process may be adopted: raw water aeration—single-stage filtration for iron and manganese removal.
c. When the iron or manganese content in the raw water exceeds the above values, the process should be determined through testing; the following process may be adopted: raw water aeration—oxidation—primary filtration for iron removal—secondary filtration for manganese removal.
d. When iron removal is affected by silicates, the process should be determined through testing.

If necessary, the following may be adopted: raw water aeration—primary filtration for iron removal (contact oxidation)—aeration—secondary filtration for manganese removal. ③ The pH of the feed water to the manganese removal filter should preferably be 7.5 or higher, and the iron content of the feed water to the secondary filtration manganese removal filter should preferably be controlled below 0.5 mg/L.