Low energy zero sludge system
Simple three-cell aerated systems deliver high quality effluent at a fraction of typical WwTP costs says David Martin, senior process engineer with LAS International.
World-wide the demand for reliable and robust wastewater treatment is increasing, especially in developing countries. However, having wastewater treatment is only part of the equation. Keeping a new WwTP running is just as important.
The elimination of routine sludge processing, separation, handling and disposal also resolves major issues in wastewater treatment. Sludge is a future environmental liability of an unknown and incalculable size.
A technology now provides low operating costs, simple operation, and almost no maintenance — even resolving the sludge issue by self-digesting the sludge within the system. Pioneered by LAS International, the system has an impressive track record.
In White Pigeon, Michigan wind-powered aeration was adopted when the fine-bubble aerators at a WwTP started to plug.
Built in 1979, the WwTP consists of a primary aerated cell followed by aerated secondary and tertiary cells serving a community of approximately 2600 people. Two upflow clarifiers for phosphorous removal follow the final cell.
The original fine-bubble diffusers became severely restricted in performance with approximately 75% of the diffusers plugged. Technical and scientific efficiency aside, the diffusers weren’t providing sufficient oxygen without expensive and time-consuming maintenance. A lack of uniform DO throughout the cells created excessive algae blooms and limited BOD reductions, in turn requiring high levels of chemicals to compensate. The restricted diffusers also put excessive back pressure on the blowers which dramatically increased energy costs and wear.
Plant managers also wanted to improve overall effluent quality and reduce 20 years of sludge accumulation. With a simple upgrade, they were able to do just that. The net effect was a 40% reduction in energy costs and an 85% reduction in chemical costs.
The diffusers were changed to a medium bubble diffused air system to reduce maintenance and improve aeration. Engineers at LAS International reviewed overall plant design and recommended that two LAS Mark 3 wind-powered aerators/processors with 0.56kW auxiliary motor kits be installed in the primary cell and smaller wind-powered units installed in the secondary and tertiary cells. The units not only enhance DO levels in the cells, but just as important, create uniform conditions without the aggressive action of standard aerators.
It was essential to maintain biological layering in the primary cell and allow sludge to settle out for anaerobic digestion. The Mark 3s are designed to optimise the overall biological conditions, improve the DO at the sludge-water interface (thus increasing the sludge digestion rate), and stimulate maximum biological reduction rates within the cells.
Nine months after the improvements had been finished, tertiary cell effluent BOD had fallen to less than 10mg/l, SS levels were down to 20mg/l, PO4 levels were falling, and chemical use had been reduced still further. Treatment was also being achieved with 20 years of sludge accumulation in the primary cell.
This much sludge accumulation required additional processing power over and above that required by the normal influent loading in order to reduce the sludge blanket back to a normal 20-30cm depth. LAS equipment is routinely used to increase the sludge digestion rate.
Available data shows that as biological conditions continued to improve in the primary cell, effluent BOD from the final cell (but before the clarifiers) was down to <10mg/l. This rate of reduction is consistent with the LAS Aero-Fac treatment system, which is what the White Pigeon plant essentially became.
Both systems use medium-bubble diffused air aeration and the LAS Mark 3 units. The main difference is that the Aero-Fac system uses less energy, requires less maintenance, and achieves more uniform conditions in the cells. It is noteworthy, however, that even this pseudo Aero-Fac system provided superior performance compared to the previous standard aerated system.
Eleven months after completion, performance data showed that the lagoon cells alone were consistently providing <10mg/l effluent BOD, with final effluent BOD at 3-4mg/l. In September 1999, DO readings in the primary cell indicated healthy biological conditions and maximum BOD/TSS reduction rates. The sludge blanket had been reduced to <1m from about 1.5m prior to improvements.
In addition, the plant’s chemical consumption has gone from a dose of ferric chloride every three weeks to a dose every 11 months. This is with a 20-25% increase in influent flow over the last three years. The cost of chemicals has gone from $19,400 to $2400/year, a significant saving. Recent testing showed P levels in the primary cell were less than 2.0mg/l.
With a simple system change, White Pigeon has been able to extend the life of its WwTP by at least another 20 years. At the same time, it has achieved a 40% reduction in energy costs and an 85% reduction in chemical costs.
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