Heat recovery from the wastewater treatment plant outlet in a prefabricated basin, with potential use for disinfecting treated wastewater before its discharge into the receiving environment.

Abstract

The invention relates to a heat recovery device intended to be placed downstream of a wastewater treatment plant, comprising a prefabricated, elongated basin in which a heat exchanger is located. No figure

Description

Title of the invention: Heat recovery from a wastewater treatment plant outlet in a prefabricated basin with potential use for disinfecting treated wastewater before its discharge into the receiving environment

[0001] Heat recovery from treated wastewater can be achieved by contact through the walls of metal exchangers, leading to low-temperature heat transfer, with the transfer of calories, to a heat transfer water which will be conveyed to a heat pump, which will enable the production of medium-temperature heat.

[0002] In parallel, the disinfection of wastewater requires the use of a disinfectant product, generally chlorine gas or bleach, with the possibility of sufficient residence time to allow the action of the disinfectant.

[0003] In both cases, the use of a contact basin ensuring a flow as close as possible to that known as "piston flow" is necessary. The combination of the two technologies therefore has the advantage of requiring only a single contact basin. Detailed description

[0004] The contact basin is preferably made of 316L stainless steel with a parallelepiped shape, and preferably with a length-to-width ratio greater than 4. Inside the basin, three zones are isolated. The first zone receives the water to be treated and is considered the transverse distribution zone. The second zone receives one or more lines of metal heat exchangers in series along its entire length and height. The third zone collects the passing water for discharge to a receiving medium. An isothermal cover is advantageously placed on the surface in several side-by-side sections to allow for maximum heat retention.

[0005] Advantageously, the basin is prefabricated, manufactured, and equipped in a factory. It is then transported to the wastewater treatment plant to be equipped. It can be installed in a pit dug for this purpose, preferably with a depth that allows for gravity flow of the treated wastewater. Its connection is simple: upstream to the wastewater, and downstream to the discharge pipe. The heat transfer pipes are connected to the inlet and outlet points, leading to a boiler room housing a heat pump.

[0006] In the case of combined use for wastewater disinfection, the necessary dose of chlorine is advantageously injected into the pipe leading into The contact basin should ideally be at least four diameters before the disinfectant enters the main basin. This ensures a thorough initial mixing of the water to be treated and prevents any segregation due to the water flow between the disinfectant and the water.

[0007] Introduction

[0008] Prefabricating a basin to receive plate heat exchangers, with installation consisting only of creating a simple pit to receive said basin, is particularly advantageous.

[0009] Using such a basin to add a means of disinfecting wastewater ensures sufficient contact time between water and disinfectant to ensure the disinfection phenomenon.

[0010] Review of the phenomena

[0011] Low-temperature heat transfer through metal plates, aside from the slowing of the process due to surface biomass growth, allows for high transfer rates, depending on the type of metals used. Generally, theoretical values ​​between 5 and 10 kW / m² of surface area are obtained. Given the amount of energy carried by wastewater, significant amounts of energy can be captured to power a heat pump, which then produces higher-temperature heat, generally between 10 and 75°C.

[0012] Wastewater disinfection is governed by the Gard law according to the following formula:

[0013] - Ds / dt=kcs / (l+k'ct)

[0014] With S the survival rate at time t divided by the initial concentration of organisms, c the concentration of the disinfectant (remaining constant over time), k and k' coefficients depending on the germs and the disinfectant. This equation projects as a straight line on a log-log scale, with a slope of n and the intercept b at S=1.0. The time t' at which the disinfectant becomes significant is s equal to b / c, and the maximum disinfection rate occurs at this time.

[0015] According to Selleck, Saunier and Colins (Journal of Environmental Engineering Division - December 1978), the coefficients measured for chlorine in wastewater in the presence of ammonia nitrogen for the removal of total coliforms lead to the following practical conclusions:

[0016] n= 2.4

[0017] b = 2.5 min*mg / l

[0018] Thus, as soon as the product of the residual chlorine and the contact time exceeds 3 min*mg / L, the disinfection process begins. With 3 mg / L of residual chlorine and 1 minute of As the contact time increases, the process begins to eliminate germs. Applying these constants to the equation above leads to the following inactivation rates:

[0019] 1 U log (90% elimination): 5.5 min*mg / l

[0020] 2 U log (99%): 12 min*mg / l

[0021] 3 U log (99.9%): 28 min*mg / l

[0022] 4 U log (99.99%): 70 min*mg / l

[0023] 5 U log (99.999%): 180 min*mg / l

[0024] Generally, at the outlet of a wastewater treatment plant, the total coliform concentration is on the order of 10p5 / 100 ml, excluding membrane filtration in a secondary settling tank. Thus, a reduction of 2 to 3 U log is sufficient, requiring a residual chlorine dose of 12 to 28 min*mg / l.

[0025] The usual contact time in the contact basin for calorie recovery is approximately 10 minutes. This means that the following reductions can be achieved:

[0026] 2 U log with 10 minutes of contact and 1.2 mg / L residual

[0027] 3 U log with 2.8 mg / l residual.

[0028] Installing a large-volume underground pipeline behind this basin, providing an additional 10 minutes of contact time, would then significantly reduce the residual chlorine doses required. For 2 U log, a residual dose of 0.6 mg / L and for 3 U log, a residual dose of 1.4 mg / L would be sufficient.

[0029] In the event of the need for dechlorination before discharge, the addition of sulfur dioxide gas (SO2) would ensure the almost instantaneous reduction of residual chlorine and would ensure the discharge into the receiving environment of water free of residual chlorine.

[0030] Benefit of combined action

[0031] Heat exchange is slowed down by biomass growth. A loss of 30 to 50% in the quality of the exchange can occur. Combining the two operations, heat exchange and wastewater disinfection, ensures that there is no biomass growth on the exchangers, thus increasing the heat transfer efficiency in the system by more than 30%.

[0032]

[0033] Manufacturing description

[0034] The metal plates constituting the heat exchangers can be made of different materials, including stainless steel of all categories.

[0035] The basin receiving the heat exchanger plates is preferably made of 316L stainless steel. Reinforcements necessary to ensure its rigidity are installed around its perimeter. Similarly, the installation of a thermal protection coating, generally of the polyurethane type, or compressed glass wool in panels, will be advantageously carried out in order to limit untimely heat loss where there would be contact between the basin and the ground.

[0036] The basin can be equipped with aeration ramps placed between the rows of exchangers, to allow the resuspension of suspended matter, continuously or intermittently.

[0037] The basin can be equipped, as an alternative to air ramps, with a brush system ensuring the sweeping of the exchangers in order to ensure their cleaning in a continuous or alternative manner.

[0038] The cover over the basin is preferably made of different types of materials, depending on the requirements. This material could be aluminum with two surfaces allowing for the placement of insulating material between the two sheets. It could also be stainless steel, or even PVC, polyethylene, or wood.

[0039] Of course, the invention is not limited to the examples just described. On the contrary, the invention is defined by the following claims.

[0040] It will indeed appear to a person skilled in the art that various modifications can be made to the embodiments described above, in the light of the teaching which has just been disclosed to him.

Claims

Demands

1. Device intended to be placed downstream of a wastewater treatment plant for the treatment of wastewater, characterized in that it comprises a prefabricated basin of elongated shape in which a heat exchanger is placed.

2. Device according to claim 1, characterized in that the basin has a length-to-width ratio greater than 4.

3. Device according to claim 1 or 2, characterized in that the exchanger is a water / water exchanger, preferably a plate exchanger.

4. Device according to any one of claims 1 to 3, characterized in that the basin comprises a first zone for receiving the water to be treated forming a transverse distribution zone, then a second zone receiving over its entire length and height one or more lines of metallic exchangers in series, then a third zone ensuring the recovery of the passing water to be evacuated to a receiving medium,

5. Device according to any one of claims 1 to 4, characterized in that it comprises an isothermal cover placed on the surface of the water in the basin, preferably in several elements side by side.

6. Device according to any one of claims 1 to 5, characterized in that it comprises a pipe for the arrival of wastewater into the basin and means for injecting a disinfectant into said pipe, preferably at least four diameters from said pipe to its arrival in said basin.

7. Device according to any one of claims 1 to 6 characterized in that it is buried.

8. Wastewater treatment plant, characterized in that it comprises a device according to any one of claims 1 to 7.

9. Wastewater treatment process characterized in comprising the use of a device according to any one of claims 1 to 7.