BUCHAREST, Sept 20 (Reuters) - Romanian port operators in the Black Sea port of Constanta are investing to boost their grain handling capacity as Ukraine ramps up exports through its neighbour, the manager of operator Comvex said.

Ukraine is one of the world's biggest grain exporters and since Russia's invasion last year, Constanta has become Kyiv's largest alternative export route, with grains arriving by road, rail or barge across the Danube.

The Romanian government has said it aims to double the monthly transit capacity for Ukrainian grain through its Constanta port to four million metric tons in the coming months from virtually zero before the Russian war started.

In August, roughly 2.7 million tons of Ukrainian grain passed through Constanta, Comvex manager Viorel Panait said in an interview with Reuters.

Since July, when Moscow abandoned a deal that lifted a de facto Russian blockade of Ukraine's Black Sea ports, Russia has repeatedly struck Ukrainian river ports that lie across the Danube from European Union and NATO member Romania.

"You cannot count on a single means of transportation because this leaves you vulnerable, as we have seen after the attacks on Ukraine's Danube ports," Panait said.

"You need truck, train and barge transport to converge."

Comvex, which doubled its barge unloading capacity last year and is currently in the process of raising its grain storage capacity by 25% to 250,000 tonnes, is now working on increasing its train processing speeds, Panait said.

It has partnered with rail operator Grup Feroviar Roman (GFR) on a new train reloading system at the Ukrainian-Romanian border crossing of Vadul Siret-Dornesti.

Panait said the system, which will become operational by mid-October, will enable Comvex to handle an additional 120,000 tons of grains per month. The operator is also adding a second automated quality control system, doubling its train processing capacity.

Comvex is also looking at a plan to unload grain from trains to barges on the Moldovan bridge of Giurgiulesti.

LOGISTICS CAPACITY

Freight logistics group TTS, which handles agricultural products, minerals and chemicals on the Danube river, completed the takeover of Constanta port solid bulk cargo operator Decirom S.A. earlier this year.

TTS director for investor relations Gabriel Techera said earlier this week the group, which has an investment budget of 80 million euros ($85.58 million) in 2023, added two floating cranes to its operations in early September.

Port operator SOCEP S.A. said it had paid 10 million euros for new equipment at its grain terminal silo and ship loader.

Panait, who is also president of the Constanta Port Business Association, said some port operators are using a digital system, which has reduced the time required to process customs statements to 30 minutes from up to 48 hours.

"I would say more than half of the port's 13 grain operators are already authorized or in the process of being so."

A government source told Reuters Constanta now had a logistics capacity of 40 million tons of grains per year, sharply higher than its annual record high 25 million tons set in 2021. ($1 = 0.9348 euros) (Reporting by Luiza Ilie; Editing by Sharon Singleton)