Difference between gray and black water treatment
Gray water is the water collected from domestic uses except for that arising from toilets, where it is called black water. The average gray water production per capita varies from country to country; Where the percentage ranges from 90 to 150 liters per day, according to age, gender, living standards, habits, lifestyle, and the degree of water availability. Gray water and black water treatment are excellent solutions to many of today’s water problems.
The climate also influences the production of gray water. For example, in a hot country, the average gray water production is about 150 liters per person per day. It is equivalent to 82% of the total freshwater consumption. 56% of which is from showering, 30% from the kitchen, 8 % from laundry, and 6% from other consumption such as rinsing, for example.
Blackwater
It represents all wastewater i.e. gray water as well as toilet water.
The names “black and gray” come from the color of the water, as gray water is somewhat gray in color. While the wastewater carrying fecal matter turns black after a short period. Here you will find more information about gray water and black water.
Black water treatment
Black water treatment results in a much larger amount of reusable water.
Another advantage of black water treatment over gray water treatment is that there is no need to connect sewers to sewage
Disadvantages of black water treatment
It is the treatment sludge that is produced in the treatment process that needs to be removed and a certain level of unwillingness to reuse wastewater based on people’s perceptions.
Although treated wastewater is generally of lower quality than rainwater. However, it is clear, odorless, and well-suited for flushing toilets, water art, and irrigation.
As a rule, they contain a higher percentage of nutrients, which enhances their suitability for plants. If both rainwater and treated wastewater are used, it is better to use rainwater for higher-grade purposes.
Risks and practices related to greywater management around the world
Gray water management usually receives less attention than other environmental sanitation systems such as solid waste management and toilet water management. In urban and peri-urban areas of low- and middle-income countries, gray water is discharged without treatment into rainwater drainage systems or sewage systems, if present. This leads to oxygen depletion, turbidity, and nutrient overload in water systems.
Risks related to poor greywater management systems
Gray water reuse for irrigation of home gardens or agricultural lands is common, especially in areas where water is scarce or water prices are high. In certain parts of the Middle East, Africa, and Latin (South) America. Hence the importance and value of gray water, but we often neglect the potential negative effects of using gray water sources. Untreated gray water, although it contains the lowest percentage of pollutants, contains pathogens, chemicals, salts, oils, fats, and solid particles that may affect human health and the quality of soil and groundwater. However, we can reduce the risks of groundwater pollution by following some basic rules.
Gray water reuse
Due to the lack of fresh water, some countries have developed different gray water reuse practices. For example, gray water reuse in Los Angeles saves from 12% to 65% of the freshwater used for irrigation. Other research has found that using greywater to flush the toilet can save between 29 and 35% of the water consumed for drinking. Treated gray water can also be mixed with collected rainwater to increase the possibilities of use.
Gray water’s physical properties
Temperature
The gray water temperature is often higher than the temperature of the water supply sources and varies in a range of 18-30°C. This difference is due to the use of warm water when cleaning and preparing food.
Elevated temperatures can increase bacterial growth. It also leads to a decrease in the solubility of calcium carbonate, which causes precipitation on the walls of tanks and pipes.
Suspended solids
The presence of food residues, fatty materials, and fibers can lead to an increase in the percentage of solids in gray water, and the presence of particles and colloidal materials leads to an increase in water turbidity and salt deposition in pipes, pumps, and filter used in treatment processes.
Chemical properties of gray water
pH and alkalinity of gray water
The pH determines the degree of acidity and alkalinity of the water. And in order to facilitate treatment and avoid negative effects when using gray water, the pH value should range between 8.4-6.5.
Salinity and SAR sodium adsorption ratio
Gray water also contains salts which can be expressed as electrical conductivity (EC). Electrical conductivity is used to measure the degree of salinity of all dissolved ions in gray water, including positive and negative ions. The highest common salts in gray water are sodium chloride, nitrates, and phosphates found in detergents and washing powders. The conductivity ranges between 1500/300 micro Siemens and may go up to 2700 micro Siemens.
Biologically and Chemically Demand Oxygen (COD, BOD)
The percentage of oxygen required biologically and chemically expresses the value of organic pollution in gray water. Where the value of chemically required oxygen expresses the amount of oxygen needed to oxidize the organic substances present in the gray water. Whereas, the value of biologically required oxygen expresses the amount of oxygen consumed for the process of respiration of bacteria in a specified period, usually five days. The biologically required oxygen value may reach 20-50 mg/l per day. While the ratio between the chemically required oxygen to the biologically required is 2-2.5.
Nutrients (phosphorus, nitrogen)
Gray water is often lower in nutrients than the toilet and sewage water. Although the nutrients saturated with nitrogen and phosphorus are important because they contain a high percentage of fertilizers that the soil benefits from in the case of irrigation. However, we must take into account the negative effects that could harm the aquatic environment. Especially the high percentage of phosphorus can lead to algae growth on the water’s surface. Nitrogen levels are relatively low in greywater, and urine and kitchen wastewater are the main culprits. Followed by toilet water and wash basins. Typical values for nitrogen in domestic gray water range from 5-50 mg/L. In countries where cleaning materials containing phosphorus are not banned, the concentration of phosphorus may reach 4-40 mg/l.
Gray water treatment
Two methods have been studied for greywater treatment:
Artificial wetland method:
This method is suitable for detached buildings and houses that have available land around them. That the required area is about 0.8 square meters per person. This method depends on plants, where the roots of plants absorb pollutants and organic matter. The water is then collected and sterilized to be used to flush toilets.
Biological filter method:
A biological filter is a commercial product that is sold as a single block. Many international companies produce such systems that treat greywater in four steps: sedimentation, filtration, adsorption, and sterilization via ultraviolet light or chemicals. These systems do not require any intervention from the owner and provide water of acceptable quality for domestic use.
