CLOSED-LOOP MUNICIPAL WASTEWATER RECYCLING SYSTEM

FIGURE 1

friendly, organic fertilizer which could be used for landscaping , for growing crops and vegetables.  The PrecipRO^TM system is based on a combination reverse osmosis and precipitation technology which in itself is environmentally friendly, since it avoids the use of additional chemicals.

Typical wastewater recovered by the Blue Spring PrecipRO^TM system meets the drinking water standards established by W.H.O. and by U.S. E.P.A.  Even though such recycled municipal wastewater may not be appealing for human consumption, it can find plenty of non-potable uses such as flushing toilets, industrial water, water for power plants, water for agriculture and landscaping.

Grey water which is filtered, treated municipal wastewater,  has been used in the past for landscaping and some industrial purposes. Blue Spring PrecipRO^TM technology is far advanced compared with the grey-water recycling technology in the sense that the grey-water does not meet drinking water standards whereas the PrecipRO^TM recycled water does.

Blue Spring's patented PrecipRO^TM technology is also superior to ordinary RO technology because the conventional RO technology is not able to recover 90% of wastewater without fouling the RO membranes. Such high recoveries of water reduce the overall cost of water, as it is delivered to the consumer.

HOW IT WORKS
Figure 1 depicts the closed-loop municipal wastewater recycling scheme. Let us say the township uses 300,000 gallons of  per day for various purposes.  Typically, 77,778 GPD will be used for potable purposes in the household, rest of it being used for non-potable purposes such as flushing toilets, industrial and commercial purposes and for landscaping and agricultural uses.

Without wastewater recycling,  the net water intake from the natural resources would be 300,000 GPD.  According to the Blue Spring PrecipRO^TM closed-loop municipal wastewater recycling scheme, the net water consumption is reduced to makeup water only, which is 81,778 GPD. This amounts to net recovery factor of  72.3% based on the total water consumption. Of these total requirements, 42,321 GPD is lost irreversibly by evaporation and another 31,222 GPD is lost due to ground seepage.  These irreversible losses can not be rectified by any wastewater recycling scheme, but they could be reduced by more careful use of water.  Furthermore, 4,535 GPD is lost in the sludge produced by the municipal wastewater treatment plant. If one discounts these irreversible losses, then the effective water recovery factor for the Blue Spring PrecipRO^TM system is 90%. 

The makeup water for the closed-loop wastewater recycling system is obtained from natural resources such as a river, a reservoir, or a groundwater well. The raw makeup water is treated by a municipal water treatment plant to improve the aesthetic quality of water and it is disinfected to protect public health. A small amount of water is lost by evaporation in the open tanks which are commonly used in water treatment plants. In coastal communities, the municipal water treatment plant may be replaced by a Blue Spring Series SW sea water desalination plant. If brackish groundwater is used, then the  municipal water treatment plant may be replaced by a Blue Spring Series BW brackish water desalination plant.

The makeup water of natural origin is distributed to the homes for drinking, cooking, showers and laundry uses.  These uses normally comprise approximately 40%-50% of the total household water consumption, the balance being used to flush toilets. The toilets are connected to the recycled water supply, using a separate pipeline.  The new homes of the future will have to be internally plumbed with two separate piping networks; one for fresh water and the other for recycled water.

The effluent from the conventional municipal wastewater treatment plant becomes the feedtock for the wastewater recycling plant.  It is first disinfected with UV lights to prevent gross contamination of the reverse osmosis membranes with bacteria and parasitic micro-organisms.  Fine particulates leftover from the wastewater treatment plant are removed in a clarifier, from which they may be sent to landfill, along with the sludge from the wastewater treatment plant.

The wastewater recycling plant uses a reverse osmosis device which splits the clarified wastewater into two streams: a purified, recycled water stream and a concentrate stream. In the above example, 200,000 GPD of purified, recycled water is available for immmediate use, after normal chorination or ozonation process. This recycled water is distributed to the households  and businesses by means of a separate, recycled water piping network. Excess recycled water may be used for landscaping purposes.  In the households, this recycled water can be used for all non-potable purposes such as flushing toilets, industrial, commercial, agricultural and landscaping purposes.

The concentrate stream which is available in the amount of  22,222 GPD is rich in natural fertilizers such as nitrates and organics. Typically, the nitrates will be 300 mg/L and organics will be of the order of  1,500-3,000 mg/L. This water is ideal for landscaping, irrigation and for agriculture. Certain plants and crops may not like the high salinity of this water, which is about 15,000 mg/L.

The industrial/commercial sector is an ideal consumer of the high-quality recycled water, since it has less hardness, less particulates and less salinity than most natural waters. Typical commercial uses include:  rinsing equipment, car-washes, power-plant steam generation, commercial laundries, cooling towers, chemical plants.

The wastewater from households and commercial establishments is combined in the sewer line which leads to the conventional municipal wastewater treatment plant. Only a high-performance municipal wastewater treatment plant should be deployed, to make sure that the effluent wastewater meets EPA guidelines. These guidelines call for BOD less than 30 mg/L and TSS less than 30 mg/L on a 30 day average.  Heavy metals and other toxic chemicals should also be below the EPA guidelines. Chlorine or ozone should not be used to disinfect the effluent, because excess chlorine and ozone are harmful to the reverse osmosis membranes used in the wastewater recycling plant downstream from the municipal wastewater treatment plant. The wastewater recycling plant includes heavy dosage of UV light to disinfect the water before it receives any further treatment. If the wastewater recycling plant is located far away from the municipal wastewater treatment plant, then UV pre-treatment should be applied to the effluent of the municipal wastewater treatment plant. For best economy, the two plants should be located close to each other.

When a certain business pollutes the wastewater with  toxic ingredients such as heavy metals, arsenic or industrial chemicals, the wastewater should be locally treated by the  business, to remove these toxic components to a reasonable level before discharging the wastewater into the industrial sewer.  Otherwise, there is a danger of accumulation of these toxic substances in the local soil and groundwater, through the percolation process.

ECONOMICS
Table 1 summarizes the cost factors involved in recycling municipal wastewater according to the Blue Spring PrecipRO^TM scheme.

The cost of electricity is assumed to be 0.07 US$/kWh. Other costs are based on typical 24 hours daily operation for 350 days per year.  Depreciation is based on straight-line method. Acceptable R.O.I. (return on investment) is

based on 8% per year. These cost factors may vary from location to location.

The total water recovery used in the calculations does not account for the fertilizer-grade byproduct of wastewater recycling plant. The unit cost of recycled water would be lower if credit were given to this by-product.

Municipal effluent water is assumed to be free of charge. Actually, credit is due for savings in wastewater disposal costs. This could be pumping cost to the disposal site, environmental fees, lost business taxes from fishing industry, etc.

The pre-profit cost of water is the all-inclusive cost which is equivalent of a selling price without profit which is typically charged by non-profit co-operative utility companies.

COST COMPARISON
For the purpose of cost comparison, the operating costs for the municipal wastewater treatment plant is ignored, since it will be common for all cases under consideration.  Typically, the municipalities pass on the operating costs of wastewater treatment to the consumers. An exception is the cost of pumping the effluent to the nearest creek or oceanfront which is usually about 0.15 US$/m3 of effluent discharge. When the effluent water is recycled, this pumping cost is eliminated.

Let us say the water authority who develops reservoirs and  lays out pipelines charges 0.40 US$/m3 for the raw water. The municipal water treatment plant adds 0.36 US$/m3 to cover the operating costs, wages, equipment depreciation and R.O.I. costs. The fresh water without recycling would cost 0.76 US$/m3 to the consumers, that is,  315,227 US$/year.  The effluent pumping costs would add another 46,724 US$/year. The total annual water treatment budget of the township, including effluent discharge costs would be 361,951 US$/year, based on 365 days of water consumption throughout the year.

The PrecipRO^TM wastewater recycling attachment to their municipal wastewater treatment plant would cost 131,515 US$ annually to operate, based on 350 days of operation per year. The fresh water treatment bill will be reduced to  82,397 US$/year. The wastewater pumping costs will be reduced to 4,419 US$/year, mainly for the fertilizer-grade water distribution. During the maintenance period for the recycling plant, fresh water consumption will be at the original level, costing 14,880 US$.  The total annual water budget including wastewater discharge will be reduced to  233,211 US$/year, based on 365 days of water consumption and 350 days of water recycling per year. Credit should be given for the 44 tons of fertilizer worth approximately US$ 13,500 that would be  recovered in the wastewater recycling process, bringing down the annual budget to 219,711 US$/year.

Thus, the township will be able to save 142,240 US$/year which is nearly 40% savings, due to closed-loop recycling of  wastewater and using it wisely. A typical household who consumes 173 m³ of water per year would save  37.32 US$/year.

Note that these cost figures include financial costs, which makes them more realistic. Also, the cost comparison does not include the positive environmental impact created by the wastewater recycling program.  It is difficult to place a cost figure on environmental and health damage caused by wastewater pollution.

Now let us compare the cost figures with conventional reverse osmosis technology which is able to recover about 55% of water instead of 90%. Let us assume that the cost of recycled water remains the same.  In this case, the net intake of natural water would be 175,742 GPD and the total annual cost of water management would be  293,879 US$/year. This leaves savings of only 68,072 US$/year as opposed to 142,240 US$/year for the PrecipRO^TM technology.

Recycling municipal wastewater not only makes environmental sense, but it definitely makes financial sense. Recycling municipal wastewater with Blue Spring PrecipRO^TM closed-loop water recycling technology makes even more sense.  It should be pointed out that in arid areas of the world, water recycling is not a matter of cost savings or environmental taboo but it is matter of survival for the population.

WATER PRICING
It is suggested that a three-tier price structure be established for the water consumers.  Consumer prices should reflect actual costs as close as possible. According to this principle, the treated fresh water would be priced 0.76 US$/m3, the recycled water would be priced 0.50 US$/m3. The fertilizer-grade water should be free or it could be sold for the cost of pumping, because it is a by-product of the wastewater recycling process.

BLENDING
Often, the actual pattern of usage of water for indicated categories will differ from the availability of water as indicated in Figure 1.  Therefore, re-distribution of some of the available water resources will be necessary.  For example, some of the natural water may have to be diverted for landscaping use, during summer months.  In some instances, there may be excess availability of recycled water and scarcity of potable fresh water. A blending scheme for the two grades of water would be highly desirable. 

Blending of recycled municipal water with domestic water presents considerable psychological hurdle and some safety risk, in case of malfunction of the water recycling plant.  There is also a question of  the possible accumulation of impurities in the recycled water. There is also a lack of  historical data on long term health effects of  drinking artificially purified water. 

The safest way to blend recycled municipal wastewater is to inject it into the groundwater basin and pump it out when need arises. There is considerable flexibility in re-distribution of the available grades of water.  When recycled water is used to recharge the groundwater basin, the quality of the pumped water should be monitored zealously.  Non-degradable impurities, particularly nitrates, can accumulate in a closed loop water recycling system.

For those who would simply walk away from the proposition of drinking recycled municipal wastewater, it may be pointed out that in reality, considerable amounts of treated municipal wastewater from various municipal wastewater treatment facilities does seep into the public drinking water supplies.  In this proposed scheme, the wastewater gets purified  by reverse osmosis which is considered the most reliable water purification process, before it is blended with the natural water.  Therefore, the problem is that of perception of the facts rather than of the facts themselves.  Until the public perception of the quality of drinking water becomes more realistic,  it is best to restrict the recycled municipal waste water to non-potable uses such as flushing  toilets, landscaping, industrial uses, etc. 

PLANNING GUIDELINES

1.  Plan ahead for water resources.  For planning purposes, determine the fresh water needs of a community.  As a rule of thumb, allow 45 gallons of water per person per day for household consumption.  Typical breakdown of water consumption in America is as follows:

Toilets:  7 GPD per person.
Showers: 8 GPD per person.  Bath-tub:  40 GPD per person.
Clothes washer and dish-washer:  4 GPD per person.
Kitchen , bathroom  (drinking, cooking and personal hygiene):  4 GPD per person.
Landscaping around residential areas:  6 GPD per person.
Commercial establishments, public landscaping:  5-25 GPD per resident.

For water-poor countries, the per capita daily water consumption may be many times lower.

2.  Keep fresh water plumbing separate from recycled water plumbing inside households as well as the municipal pipelines.  Sinks, showers, bath-tubs, laundry outlet, dishwasher, should be served with fresh water outlets. Toilets should be served with recycled water.  Swimming pools, commercial Laundromats can have their own water recycling scheme. Such scheme is based on filtration and partial reverse osmosis purification.  A recycled water line should serve commercial and industrial  businesses.  A blended supply of  fertilizer-grade water and recycled water may be used for irrigation and  landscaping purposes.

3.  Three reservoirs will be required:  one for treated fresh water, one for recycled water, and one for water for irrigation.  The reservoirs should be enclosed type, to reduce evaporation losses. For large reservoirs, this may not be practical.

4. In most likelihood,  there will be more demand  for fresh water than recycled water in the residential neighborhood, whereas the reverse will be true for industrial and commercial sector. A balanced community should consist of  both residential and industrial developments, to make better use of the recycled water resource..

5. Use the schematic shown in Figure 1 as a planning guideline. The GPD quantities displayed on the schematic diagram may be scaled up or down as necessary, to accommodate different population and different mix of water consumption. The water quantities must be balanced, and they must be achievable by the water recycling processes to be used.

6. Every municipal wastewater differs in quality, which  gives variability in the quality of the recycled water and the overall economics. A pilot plant study should be undertaken whenever possible, to evaluate the operating  parameters, cost parameters and quality parameters, before undertaking a full-scale project.

ACCUMULATION OF IMPURITIES

An important consideration in any recycling process is the possible accumulation of inert impurities.  Inert impurities are defined as those impurities which are not significantly removed by the purification process. 

In the schematic shown, a small amount of water being purged into the soil effectively limits the concentration of the inerts in the recycled water. Some inerts, like heavy metals, will accumulate in the soil.  There are three precautions to be taken. First of all, set discharge limits for the poisonous inerts, on industrial establishments. Second, monitor the poisonous inerts in the soil and in the ground water. Third, when accumulation of inerts is detected, take measures to extract them from the recycled water at the industrial source of wastewater. Specialized PrecipRO^TM system is available for extraction of most poisonous impurities, including arsenic and mercury.